A Complete Guide to Understanding the Immune System

Understanding the Immune System

understanding the immune system
Mary Shomon

The immune system is our frontline protection against disease and many painful, and damaging, physical conditions.  Our immune function is a system, a complex biology made possible by the elegant interaction of many parts that protect us against invaders and disease of all kinds.

Each of those parts is important—and plays an active role in protecting your health and well being.  In this special About.com Thyroid Disease site section, we take a look at the immune system, its anatomy, disorders, and other points of interest.

With information from the National Cancer Institute, National Institute of Allergy and Infectious Diseases, and other online resources, I hope this guide leaves you better informed about immune and autoimmune conditions, including those of concern to thyroid patients.

Throughout this guide, you’ll find easy reference links to a glossary of terms and other authoritative links for further reading.​

As with any issue involving your health—education and advocacy are key.  Always ask questions to get the answers you need to make your own, informed, healthcare decisions.

Introduction to the Immune System

With the job to deter, prevent, and eradicate infection and disease, the immune system plays an important role in every day of our lives.  As a complex network of specialized cells and organs, the immune system defends the body against matter and molecules it identifies as foreign.

Foreign invaders could be bacteria, viruses, fungi, and parasites.  But when the immune system malfunctions, the body is left vulnerable to attack from itself—from diseases like allergies, arthritis, AIDS and cancer.

Inside and out, microbes outnumber us. Like humans, microbes—both good and bad—seek to reproduce. Our helpful gut bacteria allow us to digest food, and aid in immune function.  But when harmful pathogens enter the body, it is the immune system that sounds the alarm and triggers a defensive response.

The Immune System: Complex, Adaptive, and Working for You!

The immune system is equal to the brain and the nervous system in complexity.  Although we routinely take our immune function for granted, it has some amazing qualities that include:

  • Like peering into a mirror, the immune system has the ability to identify itself in an environment where foreign bodies are present. Similarly, immune function recognizes what does not belong in the reflection—the non-self—or foreign microbes in our own bodies.
  • Like a scrapbook, the immune system carries memories of the chicken pox you had in second grade. Because of that memory, you won’t get the chicken pox again as long as your immune system functions properly.
  • Drawing on an extremely diverse ecosystem, the immune system rapidly recognizes non-self microbes—intruders—and readily applies a solution from a rich away of defensive processes.

The success of this ecosystem depends on a dynamic communications network of millions of cells. Signaling between groups and subsets, the network, like a beehive, is prompt and effective as it carries out its regulatory, healing, and defensive duties—every day of your life. 

Self And Non-Self: Understanding the Immune System

self and non-self, immune system
National Cancer Institute/ NIAID

One of the most basic human instincts is that of “us,” and “them.”  Recognizing friends—and enemies—is key to survival.  Your immune system functions the same way.

A fundamental ability of the immune system is to recognize itself—and to identify its non-self.  Almost every cell in your body carries a marker, like a nametag, that identifies it as part of you.

Your millions of cells and friendly microbes live together in a state known as self-tolerance. This peaceable existence makes sense, and your immune system does not usually attack cells that identify themselves as part of your crew.

It is another story altogether when a substance, or microbe, cannot identify itself as part of the greater good.  When the immune system recognizes an intruder, it naturally mounts an immune response.

What Is an Antigen?

Intruders or substances that provoke a response by your immune system can be either antigens, or allergens.  Consider features of both:

  • Foreign bacteria, viral matter, fungi, and parasites are considered antigens. A product, or part, of one of these antigens could also trigger an immune response.
  • The immune system identifies foreign matter through its epitope, an intricate type of handle that protrudes from an antigen. Once identified, immune cells attach to the antigen at its epitope.
  • A microbe may have several kinds, or even several hundred, epitopes.  Some epitopes provoke a bigger response from the immune system.
  • This response is problematic for transplant patients, whose bodies may reject a transplanted organ, or tissue, because their immune system identifies it as an antigen.  The only human transplant tissue not considered an antigen, would be that from an identical twin.
  • Improperly digested proteins can also be rejected by the immune system if they are not broken down by the body into non-antigenic building blocks.
  • Oftentimes, the immune system identifies a harmless material—like ragweed pollen—as an antigen.  This allergen, by itself, has no effect on the body, but the recognition of it as an antigen by the immune system sets off a host of symptoms that we call an allergic response.

For reasons that are being studied, but not yet understood, the immune system sometimes identifies molecules of the self, as non-self.  This case of mistaken identity leads the immune system to attack itself, in what is termed an autoimmune disease.  There are many autoimmune conditions like rheumatoid arthritis, systemic lupus erythematosus, or diabetes, Type 1.

Genes and Markers of Self: What's in a Name?

The wild and wonderful immune system is a fascinating place, home to things with large names that have very important functions.  One of those larger names is major histocompatibility complex (MHC).

First described by a British immunologist in 1936, MHC is a group of genes that that have a big impact on the immune system of vertebrates—animals that have a backbone.  The prefix “histo” refers to tissue.  Remember a gene is the basic component of heredity, and is composed of DNA. So this special group of genes is concerned with the compatibility of all that relates to tissue.

MHC was first identified during investigation of tissue transplants.  The information coded in this group of genes varies widely from person to person. This diversity is called polymorphism, meaning many shapes, or forms.  It is the great difference in gene shapes between individuals that make organ and tissue transplants so difficult—and why so many organ transplants fail.

What Does MHC Have to Do With Immune Function?

MHC has a specialized purpose in the immune system.  MHC gene molecules bond, or tie up, products of pathogens—foreign invaders in body.  It goes something like this:

  • MHC attaches to the peptides secreted by a pathogen.  This action is displayed on the surface of the pathogen, or infected cell.  This signals the immune system to send help—and it does.
  • Once notified by the MHC, the immune system sends solutions based on the type of foreign invader.
  • Pathogens that invade cells are attacked by a type of white blood cell called a T-cell.  Macrophages hunt down bacteria, and B-cells produce antibodies to deter and destroy pathogens outside of cells—called extracellular pathogens.
  • MHC also helps B-cells, T cells, and macrophages to recognize and communicate with each other.
  • The various molecules of the MHC are derived from several different types of genes, which increase its complexity and ability to mediate immune function through the use of Helper T-cells, and Killer T-cells.
  • Killer T-cells attack body cells decimated by viral attack, or transformed by cancer.  Helper T-cells are a core feature of adaptive immunity and help activate B-cells and T-cells to eliminate infected cells.
  • Helper T-cells, and Killer T-cells on MHC molecules form a dual system for recognizing and managing threat from pathogens in the body.  This form of recognition is called MHC restriction.

MHC molecules play an essential role in detection, signaling, and stimulation of immune response.  In other words, healthy gene function is crucial to healthy immune function.

The Anatomy of the Immune System

anatomy of the immune system
National Cancer Institute/ NIAID

Whether you cut your finger or scrape a foot, your immune system responds on site to prevent infection and deter disease.  This ability is supported by specific structures located throughout your body.

Immune System: Basics

The immune system is populated by lymphoid organs, nodes, glands, and other tissue that support your growth and development.  These structures are also key players in the deployment of lymphocytes, the white cells that act as first responders when you are injured or ill.  Lymphoid organs include:

  • Bone marrow:  All blood cells, and cells destined for immune function, are produced in the bone marrow, soft tissue located in the hollow shafts of long bones. Stem cells produced in bone marrow differentiate to take on different functions, like lymphocytes, and pathogen-devouring phagocytes.
  • Thymus:  Located behind the breastbone, the thymus is a multi-lobed organ involved with the maturation, education, and function of T-cells.  Upon arrival from the bone marrow, immature T-cells become immunocompetent by gaining ability to distinguish between self and non-self.
  • Lymph nodes:  Clustered in the neck, armpits, abdomen, and groin, lymph nodes house B-cells, T-cells, and other cells needed to initiate a successful immune response. Working as a communication center, each lymph node constantly samples material moving through the body for pathogens, or foreign invaders, that require an immune response.  When you visit your doctor, they may check for swollen lymph glands, indicating an active immune response.
  • Spleen:  Behind the stomach, the spleen contains lymphoid tissue that processes and renews blood and immune cells.  Microphages operating in the spleen trap microorganisms collected by blood cells circulating throughout the body.  If you lose your spleen through disease or injury, you are at higher risk for infection.
  • Tonsils and adenoids:  As part of the lymphatic system, your tonsils and adenoids act as sentries, sampling particles that enter your nose and mouth for pathogens.
  • Appendix:  While considered a vestigial organ, the appendix may play an important role in maintaining a healthy ecosystem of microbiota, aiding in immune health.
  • Peyer's patches:  These small clumps of lymphoid tissue are located along the small intestine.
  • Lymphatic system:  Traveling between lymph nodes, the lymphatic system is a highway between vessels, tissues, nodes, and organs that carry lymph, a colorless fluid filled with white blood cells.  Lymph bathes body tissues and mediates transportation of antigens to immune cells.

A Closer Look at Lymphocytes

Lymphocytes are small white blood cells circulating throughout the lymphatic system.  The two major classes of lymphocytes are B-cells and T-cells.  B-cells remain in the bone marrow to mature, while T-cells travel to the thymus to complete their education.

Once they are mature, B-cells and T-cells use the bloodstream and lymphatic system to travel continuously to lymphatic organs, lymph nodes, and other sites in the body.

A major function of the immune system is the sampling and managing potential pathogens.  Points to consider along the way include: 

  • They are everywhere:  Clusters of lymphoid tissue are found throughout the body.  Immune cells are located within layers of your skin, and in mucous membranes lining your respiratory and digestive tracts—areas that receive high exposure to foreign particles. Responsive tissue is also part of the tonsils, adenoids, appendix, and Peyer’s patches.
  • Lymph, lymph, and lymph:  Lymphocytes travel in lymph fluid via lymphatic vessels. Tiny lymphatic vessels empty into larger veins, eventually reaching the thoracic duct, which empties into the bloodstream.
  • And on:  Traveling continually throughout the body, its tissues, and organs, lymphocytes are forever on alert for foreign antigens.  With human and cellular age, lymphocyte function declines. Most lymphocytes are programmed to die, and are replaced by newly matured cells.

Vast and interactive, the human immune system is wherever you are, undertaking the tough job of protecting you in an often dangerous molecular world.

Cells and Secretions of the Immune System

You know the importance of getting your message across. So does the immune system.

For success, our immune system has to have solutions to many kinds of problems.  Also, good immune function requires the necessary tools, or cells, to carry out those solutions. Last, immune cells must be able to communicate with each other.

Though it seems like an easy idea, chemical messaging in the immune system is complicated—and highly effective.  Through direct contact and cellular secretions, message pathways activate to create solutions like:

  • Innate immune response: On its own, your immune cells contain genetic pattern information that allows the immune system to identify and respond to a general threat, like a virus, bacteria or other germ.
  • Adaptive immune response:  Upon signaling from immune cells of an intruder, or a potentially harmful antigen, the immune system creates site and pathogen specific cells to target the intruder.

To respond effectively, the immune system retains a set of immune cells that match an invader it has identified and managed.

When a similar pathogen enters the body, quick signaling enables your immunity to kick in—synthesizing a cellular army on your behalf to fight the intruder.  When the infection is handled, the immune response is suppressed, powering down the powerful immune reaction.

Signal, response, delivery—a healthy immune system responds to whatever comes your way.

Lymphocytes, B Cells and Antibodies: Understanding the Immune System

immune system
National Cancer Institute/ NIAID

Understanding Lymphocytes

The small white blood cells called lymphocytes are about a trillion of your closest friends.  Found in your blood, and lymph fluid, lymphocytes are go-to cells when your body is invaded by a pathogen or unfriendly microbe.

Lymphocytes carry out important functions of your immune system.  Two prominent classes of lymphocytes include B-cells and T-cells:

  • B-cells: Maturing in bone marrow and outside of the thymus, B-cells are able to detect and recognize antigens.  B-cells secrete soluble substances called antibodies into body fluid, or humors, when an antigen is detected.  This process, called humoral immunity, allows the antibody to coat, or tag, bacteria or harmful molecules.
  • T-cells:  Like B-cells, T-cells recognize and target antigens. Developed in bone marrow, but maturing within the thymus, T-cells can penetrate cells tagged by antibodies as antigens.  These antigens could be foreign particles, bacteria, fungi, or body cells transformed by malignancy.  The process whereby invading molecules and mutant cells are neutralized is called cellular immunity.

While B-cells and T-cells look similar even under a microscope, they have important differences:

  • Both types of cells have markers on their surface that distinguish their behavior among other cells
  • For thyroid patients, an important difference between B-cells and T-cells is a marker carried by T-cells called T3, or CD3.
  • Most Helper T-cells (different from Killer T-cells) carry a T4, or CD4 marker
  • Suppressor T-cells (which are specialized lymphocytes that depress production of lymph cells when an infection is managed) carry a T8 or CD8 marker.

Even in such large groups of disease and infection fighting cells, knowing the discreet, individual function of lymphocyte subsets makes a difference to understanding normal, and abnormal, immune system function.

B-Cells and Antibodies

B-cells are lymphocytes with a job to do.  That job is to recognize and provide a rapid response when an antigen they recognize comes across your path.

B-cells are antigen specific. They recognize only one general pattern, and spend their time watching and waiting for that molecule, or virus, to show up on your doorstep.  While one B-cell may recognize pneumonia, others recognize the rhinovirus (common cold)—and when they do—immune reaction occurs in short order.  How do they do it?

Chain Reaction: B-Cells, Antibodies, and Immunoglobulins

When a B-cell encounters its triggering antigen, it responds with a group of collaborating cells, like T-cells.

The B-cell response takes the form of manufacturing antibodies, special proteins designed to meet up with the intruder in your system.  Once antibody meets antigen, there are several ways the antibody can neutralize the pathogen until T-cells, macrophages, and others can arrive on the scene to finish the job.

Here is a run-down:

  • After alert of the presence of the antigen, B-cells manufacture plasma cells.  Plasma cells are essentially a portable manufacturing plant to produce a specific antibody for the newly recognized intruder.
  • The plasma cell manufactures millions of identical, or clone, antibodies to aid in the fight against the intruder.  These antibodies pour into your bloodstream.
  • Each of these antibodies pursues its matching antigen.  Sometimes it is a general fit, sometimes very specific. When the antibody binds to the antigen, the invader is tagged for attack.

Antibodies have several means of attacking their target antigen, including:

  • Antitoxins:  Interlocking with toxins secreted by bacteria, certain antibodies are able to directly disable the molecule
  • Coating:  Some antibodies locate and coat (or opsonize) microorganisms, making them attractive to scavenger cells that disable and eat the antigen.  Also, in a phenomenon known as antibody-dependent cell-mediated cytotoxicity (ADCC), cells coated with antibody become vulnerable to attack by several types of white blood cells.
  • Serum enzymes:  Enzymes (or Complement) created by the binding of antibody to antigen are lethal to the invading microbe.
  • Blocking:  In a trait used successfully to create vaccines, some antibodies block entry of a virus into cells.

Antibodies belong to the immunoglobulin family.  Interesting points about this useful group of molecules includes:

  • Immunoglobulins are proteins composed of a chain of amino acids called polypeptides.
  • Each antibody has two heavy, and two light, chains of polypeptides.
  • The two sets of chains form the characteristic “Y” shape of this protein.  The tips of the Y are formed precisely to attach to an intended target.
  • The precise shape of the “V” space within the Y form, is formed to enable the antibody to attach, or enfold, the antigen it seeks. This is called the variable region.
  • The stem of the Y allows the antibody to attach to, and be recognized by, other members of its immune group, and is called the constant region.

So far, medical science has identified nine distinct classes of human immunoglobulin (Ig) that are:

  • IgG:  Four types that that suffuse tissue spaces, coating microorganisms to quicken their uptake by other immune cells
  • Ig A:  Two types that are found in body fluids like, tears, saliva, the respiratory tract and gastrointestinal tract
  • IgM:  Existing mainly in the bloodstream, IgM combines in star-shape clusters where it kills bacteria
  • IgE:  Occurs in trace amounts as a defense against parasites, but is a major player in allergic reactions
  • IgD:  Located in B-cells, IgD works to help regulate the cell’s activation

Along with the larger host of immune system functions, B-cells are an important—and unique—means of keeping you healthy.

T Cells, Lymphokines, Cytokines, Natural Killer Cells, and Phagocytes

laboratory technician picks up a test tube with a human blood sample
David Silverman/Getty Images News/Getty Images

Like B-cells, T-cells are lymphocytes with the ability to recognize antigens, but not the same way.  B-cells recognize the structure and signature of antigens, while T-cells are able to see fragments and products of antigens lurking on the surface of infected cells.

To do this, the body employs several types of T-cells, including:

  • Helper T-cells:  These cells, with a T4 cell marker, help to induce an immune response by activating B-cells, and other T-cells, macrophages, and Natural killer (NK) T-cells.  A subset of these cells also acts to suppress immune function when the job is done.
  • Cytotoxic T-cells (CTLs):  These lymphocytes, also called Killer T-cells, carry a T8 marker, and directly attack compromised cells carrying foreign or infectious material. These cells do not recognize the pattern of an antigen, but the product of its presence on, or in, another cell.

Cytokines: Getting The Message

Communication within the immune system is conducted through direct physical contact and by chemical messaging. Produced by immune cells, cytokines are important because they trigger other cells in the immune system to act—or not to act.  In this way, cytokines mediate cell growth, direct cell traffic, and activate cells and macrophages, among other functions.

Cytokines, are “chemical messengers,” of the immune system.  Types of cytokines include:

  • Lymphokines: Lymphokines are a type of cytokine produced by T-cells.  Binding to specific receptors on target cells, lymphokines can marshal other cells, and substances, to produce an immune reaction—like an inflammatory response.
  • Monokines:  These chemical messages are produced by monocytes and macrophages.

Interferon was one of the first cytokines identified by researchers.  Interferons are a family of proteins with antiviral properties produced by T-cells, macrophages, and certain cells outside the immune system. 

As cytokines are discovered, they are termed interleukins, or messengers between leukocytes, which are white blood cells.  Consider these research findings about cytokines:

  • Some cytokines are known to activate macrophages.
  • Types of interleukin, derived from macrophages and other immune cells, have T-cell boosting qualities, nurture immature blood cells, and assist B-cells, mast cells, and granulocytes to grow and differentiate
  • Some cytokines, created by genetic engineering, are used in clinical trials with patients suffering cancers, blood disorders, and immunodeficiency diseases, like AIDS.

Researchers hope further investigation into the qualities of T-cells and cytokines leads to development of viable tools and processes to fight tumor cells, battle viruses, and disease.

Natural Killer Cells

In addition to B-cells and T-cells, another type of lymphocyte are Natural Killer (NK) cells. 

Like cytotoxic T-cells, NK cells contain granules filled with potent chemicals.  These cells are considered natural killers because they do not need to recognize a specific antigen before attacking.  Consider the following about NK cells:

  • NK cells target tumor cells and protect against a wide variety of infectious microbes.
  • In patients with immunodeficiency diseases, including AIDS, NK cell function is abnormal.
  • NK cells contribute to immunoregulation by secreting high levels of powerful lymphokines.
  • Cytotoxic T-cells and NK cells can kill on contact.  These cells bind to their target and insert chemicals that riddle the cell membrane with holes. Target cell death occurs when fluids seep out and the cell bursts.

Because of their diversity, and function, NK cells, like other immune cells, are the subject of extensive research to develop new methods to fight tumors and infection.

Phagocytes, Granulocytes, and Their Relatives

Large and lethal to antigens and foreign microorganisms, phagocytes are literally “cell eaters.” 

These robust white blood cells engulf and digest their prey, or present it to another lymphocyte for handling.

Two types of phagocytes are monocytes and macrophages. Consider these interesting points about our cell consuming friends:

  • Although they can circulate in the bloodstream, monocytes can migrate into tissue, where they transform into macrophages.  These “big eaters,” specialize to areas of the body, like the lungs, kidneys, brain and liver.
  • Macrophages scavenge the body for worn out cells and debris, and process antigens for presentation to T-cells.
  • Macrophages secrete chemicals vital to immune response, regulation, and inflammation for body defense.
  • Among the chemicals secreted by macrophages are enzymes, complement proteins, and regulatory factors like interleukin-1.
  • Macrophages also carry lymphokine receptors that allow the macrophage to pursue microbes or tumor cells.
  • Another critical phagocyte is the neutrophil. Neutrophils are both phagocytes and granulocytes—they contain chemical-filled granules. These chemicals destroy microorganisms and play a key role in acute inflammatory reactions.

Granulocytes are interesting members of the phagocyte family, for reasons that include:

  • Granulocytes are polymorphonuclear, or polymorphs (because their nuclei come in "many shapes").
  • Types of granulocytes are eosinophils, and basophils, as well as neutrophils.
  • Granulocytes kill their prey by chemically digesting them, or releasing chemicals into the intercellular environment to degrade the antigen or microbe.
  • Mast cells are a non-circulating counterpart to the basophil.  Mast cells live in the lungs, skin, tongue, and mucosal linings of the body.  Mast cell function is largely responsible for the symptoms of allergies.
  • Blood platelets also contain granules. They participate in clotting of blood and wound repair. Platelets also secrete chemicals that stimulate immune system function.

Macrophages are not the only cells that deliver antigens to lymphocytes. Other relatives with antigen-presenting cells include:

  • Irregularly shaped white blood cells called dendritic cells, found in the spleen and other lymphoid organs, have threadlike tentacles that connect lymphocytes with antigens.
  • Langerhans cells are dendritic cells that live in skin layers and capture and transport antigens to nearby lymph nodes.
  • Many other types of body cells, properly stimulated, can also be recruited to present antigens to lymphocytes.

The phagocyte group and its family members actively seek and destroy antigens and sweep up cellular debris, providing essential service to your immune system function.

The Complement System and Immune Response

Laboratory blood testing

The Complement System

Made up of about 25 proteins that complement the activity of antibodies in destroying bacteria, the complement system is a primary defense response that helps clear pathogens from our bodies.

Complement proteins are small molecules that circulate in the blood.  When triggered by the presence of an antibody interlocked with an antigen, complement molecules coordinate with other components of the immune system to perform coordinated, protective tasks that include:

  • Opsonization:  Opsonization is the process by which phagocytes bind to viral and other material, capturing and destroying the antigen.
  •  Lysis:  The action by which pores, or holes, are created in the cell membrane of a virus or antigen is called lysis.
  • Inflammation:  Formed by the release of cytokines, inflammation occurs, accelerating immune response to antigens.  While we look at the signs of inflammation during illness, or after injury, as a nuisance, the inflammatory response of redness, warmth, swelling, and pain are signs of your immune system at work.

The precise steps taken by complement proteins in carrying out these functions is called the complement cascade.  There are, at present, three known pathways toward immune defense taken by the complement system.  Those pathways are named:

By any pathway, the complementary system is an orchestrated immune function that protects your health and orients the immune system toward a new intruder, and its pattern.

Your immune system thinks globally—and acts locally—to halt damage, and restore health.

Mounting an Immune Response

Infection—you have had one, or two, or many.  Most of the time, you did not even know you had an infection, because your immune system eliminated it before you became aware.

Caused by bacteria, viruses, fungi, and parasites, infections remain the most common cause of human disease.  While a rhinovirus infection causes the common cold, other infections cause life-threatening diseases.

The immune system works from the outside in.  Skin is the first line of defense to an invading microbe.  From mucous to scavenger cells, physical and cellular obstacles lie in wait for microorganisms seeking residence in the human body.

Once inside the body, a host of general, and specific, immune responses await viral and other interlopers.  Disabled, weakened, or killed by enzymes, chemicals, or the complement system, foreign antigens face a formidable foe in the human immune system.

Antibodies and You

Freedigitalphotos.net, Mary Shomon

Elegant, efficient—and all yours.  The immune system is a vast, responsive network consistently tuned to your inner and outer environment and always sifting for intruders.  Part of that network is a group of proteins called antibodies.

Before discovery of antibodies, medical science did not understand how the immune system could recognize and quickly respond to an almost infinite number of foreign invaders, called antigens.

Today, we understand how antibodies are created, and why they are essential to healthy immune function

Understanding the Work of Antibodies

Despite the ability to respond to a world of germs, the human immune system carries a limited number of genes.

In theory, those genes cannot differentiate, or change, fast or widely enough to respond effectively to persistent foreign intruders.  And it turns out they don’t change—they create antibodies that do.

Here is how it works:

  • When facing an intruder, B-cells create plasma cells.
  • Into the plasma cells, B-cells slice and dice fragments of DNA that form a unique antibody to challenge the intruder.  DNA is a blueprint of your genetic material. Instead of creating a multi-purpose antibody, the B-cell creates just the right recipe to confront one antigen.
  • Plasma cells create the antibody as directed.
  • Millions of copies of this antibody pour out to meet the intruder, tagging and binding the antigen until the next wave of immune cells arrives.

An important job of the parent B-cell is the shuffling and composing of DNA segments to meet the specific purpose of the “Y” shaped antibody.

When the B-cell reproduces, its descendants will all have the ability to make the unique antibody it creates.  This process creates a genetic knowledge base in the body that is constantly evolving to meet new challenges—and take care of old ones.

Antibodies: The Shape of Things

The shape of immune system cells is specialized and important.  Produced by B-cells, antibodies are proteins synthesized to address a specific antigen and are shaped like a “Y.” 

Distinct functions of each region on an antibody enable it to recognize and process harmful intruders in the body.  That said, basic regions, common to all antibodies include:

  • Constant region:  The stem of the antibody
  • Variable region:  The “V” formed by the two arms of the “Y”
  • Binding sites:  The ends of the “Y” that bind with antigens
  • Polypeptide chains:  Each antibody has a light, and a heavy, chain on each side of the “Y”

How Antibodies Work

The regions of an antibody aid its work as a binding protein.  Unlike other cells in the immune system located in a specific organ or tissue, antibodies flow with the bloodstream, seeking specific antigens, or invaders, in your body.

The variable region, which forms the “V” on the antibody, is an area of action and interaction with antigens.  Interesting points include:

  • At the end of the arms marking the variable region, the binding sites can attach to an antigen.  The stem end of the antibody cannot.
  • The V-pocket of the antibody can enfold to surround an antigen
  • The variable region itself contains antigen-like pieces, that are known as idiotypes
  • Idiotypes recognize specific epitopes, or handle-like structures, on the antigen it is looking for.  Antibodies bind to their antigen through its epitopes.
  • Idiotypes can recognize and trigger complementary antibodies.  Through a continual feedback process, the immune system is signaled, tuned in, shut down, or amplified, depending on the field work of antibodies and other immune cells as they engage antigens.

The constant region that forms the stem of the “Y,” is common to all antibodies in the body.  Because the variable region, and the idiotypes on the binding tips are constantly in flux—the constant region is just that.  A constant region that is readily identifiable by immune cells as self.

Antibodies, immune cells, and their function are fruitful ground for medical science.  With the discovery of tumor-associated antigens (TAAs), researchers are looking for immune-based solutions and vaccines to use in the fight against cancer and other diseases.

Shifting, translating, and neutralizing, the unique function and shape of antibodies is a key point in the study of immunology.

Receptors for Recognizing Antigens

Like the brain, the immune system does a lot of its work through pattern recognition.  For B-cells and T-cells, receptors on these immune cells respond to pattern and proteins.

B-cells and T-cells are lymphocytes—cells washing through body tissue on the lookout for pathogens and antigens.

In order to recognize target antigens, both types of cells are enabled with special receptors on their surface.

  • B-cells promote immune health primarily by producing a specialized antibody that will neutralize a specific antigen.  On its surface, the B-cell has a receptor that is a primitive form of the antibody it will manufacture if triggered.  This receptor—when it comes in contact with its target—signals the B-cell to produce plasma cells.  The plasma cells then synthesize and release antibodies against the target antigen.
  • T-cell receptors work differently than B-cells.  While receptors on B-cells await a specific antigen, T-cell receptors are looking for parts, or products, of an antigen.  The receptors on a T-cell are located within the surface membrane of the cell.  Washing through the body, the cell receptors recognize antigens previously identified through the major histocompatibility complex (MHC) .  Unlike B-cells, T-cells can only recognize antigens that are previously tagged, or bound, by another immune cell.

The immune system is an immense example of collaboration among parts for a greater goal.  When a B-cell locates bound MHC antigen fragments, it displays the material, garnering the interest of mature Helper T-cells.

This signaling of other immune players, interaction, production of antibodies, and immune response occurs every day as you head to your job, reach for a coffee, or browse the internet.

Immunity and Vaccines

vaccines, immunity

You had the chicken pox—so it is not likely you will get them again.  You had a tetanus shot, so you are not worried about that either.  You have a natural immunity to the chicken pox, and an acquired immunity to tetanus.

Centuries ago, Greek physicians observed individuals who recovered from the plague were not troubled by the disease again.  They had acquired immunity.

Immunity is the ability of your body to fight, or ward off, an infection or other pathogen due to the memory, or training, of your immune cells.

How Does Immunity Work?

When your body comes in contact with an antigen, your immune system activates B-cells and T-cells.  As the antigen is defeated, some of these cells take on the pattern, or memory, of the antigen, in case you encounter it in the future.  When your body recognizes the same virus, or antigen, again, your immune system now recognizes the pattern and responds quickly—usually saving you the symptoms.

Even when you have immunity to a particular antigen, the strength and duration of that immunity depends on heredity, the health of your immune system, and the type, quantity, and entry point of the pathogen that challenges you.

Types of Immunity

There are different kinds of immunity, and different ways of acquiring immunity:

Passive immunity:  Infants are born with weak immune responses, but they benefit during the first months of life from antibodies conferred by their mother before birth.  Maternal antibodies cross the placenta to the fetus, and pass through breast milk after birth.  The passage of antibodies to the fetus, and to the baby, is a form of naturally acquired passive immunity.

Passive immunity also takes the form of serum prepared from individuals who recover from an illness.  Gamma globulin, or , is sometimes given to travelers to countries where hepatitis is common.

Recently, victims of the widespread Ebola outbreak were given serum containing antibodies from individuals who survived the usually fatal disease.

Active immunity:  Active immunity is conferred by actual infection by an antigen, or by vaccination. Attenuated vaccinations contain viral matter too weak to cause a full-blown illness, while inactivated vaccines contain only killed viral material.

Ongoing research investigates how biotechnology can drive development of new types of vaccines. Subunit vaccines are prepared using only antigen material—instead of the whole molecule.  Subunit vaccines are available for meningitis, pneumonia, and hepatitis B.  The aim of all vaccines is to confer immunity—without the symptoms and impact of the original disease.


Genetic engineering (GE) aids the development of processes to create new types of vaccines for a world in need of them.

The Vaccine Process

There are several methods where science uses biotechnology to create vaccines:

  • DNA vaccines:  By artificial manipulation, researchers isolate specific genes for transfer into the DNA of microbes, or animal cells.  Through this process, the manipulated microbes become factories to produce a target antigen.
  • Monoclonal antibodies: Monoclonal antibodies are synthesized and used to separate the target antigen for use in a vaccine.  By separating an antigen from its microbe, safer vaccines are developed, like the hepatitis B, or malaria vaccine.
  • Vaccinia virus: Vaccinia virus was isolated from the cowpox virus, and later used to develop a vaccination for smallpox.  Using vaccinia, researchers reengineered the material to stimulate an immune reaction to both vaccinia and components it carries in its reengineered from—like genes isolated from rabies, influenza, and AIDS molecules.
  • Chemical synthesis:  By exploring gene coding of an antigen, scientists chart the sequence of amino acids that make up a specific antigen.  Taking small areas of the molecule, researchers then assemble that area—in sequence—using chemicals. The hope is to develop a completely synthetic vaccination that is powerless to harm—but offers powerful protection.

Advances in immune function bioengineering will likely produce breathtaking—and life saving—medical advances in future years.

Immune System Disorders: Including Allergies and Autoimmune Disease

immune disorders


Pop quiz:  What is the most common type of allergy?

You answered hay fever, right?  And you would not be wrong.  Plus, the most common allergic reactions are respiratory—sneezing, runny nose, itchy eyes, asthma, and hives.

While it seems your nose and sinuses are reacting to grass pollen, or dust, it is actually your immune system, overacting to what is probably a harmless substance in the air.

Allergic reactions occur when your body produces antibodies, specifically immunoglobulin E (IgE)  in response to molecules of oak, ragweed, or other pollen. We do not yet know why the body reacts so strongly to harmless substances.  Other points about allergic reactions include:

  • IgE is originally thought to have evolved as a defense against parasitic worms
  • When exposed to allergens, the immune system of susceptible people synthesizes large quantities of IgE, for deployment to mast cells that reside in tissue, or basophils, that freely circulate 
  • Mast and basophils sensitized with IgE release a powerful chemical reaction to initiate what becomes an allergic response.  This includes wheezing, sneezing, runny eyes—even anaphylactic shock

Allergy management usually involves taking medications to deal with symptoms, or longer term management through means of desensitization to triggering allergens.

Autoimmune Diseases

Your immune system works to protect you each day of your life.  When some part of the immune system breaks down, it may fail to recognize your own cells—and attack them as if your own tissue is an invader.  This is the basis of an autoimmune disease.

What are Autoimmune Disorders?

Autoimmune disorders can target almost any body tissue or organ.  When the immune system mistakenly targets your own tissue, T-cells and antibodies that formerly attacked foreign pathogens—now attack you.

Points to consider about autoimmune diseases:

Many autoimmune diseases cause reactions in multiple areas of the body.  By producing autoantibodies, the body directs an attack against its own, or self, tissue.  Just a few types of autoimmune disorders include:

One danger of autoimmune diseases is development of immune complex diseases, where antibodies bond with self-antigens to form molecules that lodge in tissue and set off acute or chronic inflammation.

What Causes Autoimmune Diseases?

Persons with autoimmune diseases suffer varying severity and type of symptoms. For patients with Lupus, B-cells are too active, while suppressor cells are underactive.  We do not know why some people develop autoimmune diseases, but potential causes include:

  • Viruses
  • Environment exposure
  • Exposure to chemicals, or toxic chemicals
  • Damage from drugs or medication
  • Heredity

Sex hormones are thought to be a factor because women develop autoimmune diseases more commonly than male counterparts.

Treatment for autoimmune disease often involves corticosteroids, immune-suppressing medications, anticancer agents, purposefully damaging the lymph nodes with radiation, and plasmapheresis, which treats blood to remove cellular debris from the bloodstream.

As a system designed to protect our health, the immune system is a fearsome foe when turned against our own tissue.  Future medical advancements may unlock the reasons why the body turns on itself.

Immune-Complex Diseases

During immune system response, antibodies interlock with antigens.  After disabling the antigen, an antibody then presents the antigen to another immune cell, like a macrophage.

Another type of immune disorder is an immune complex disease.  When clusters of locked up antigens and antibodies are not removed through the spleen and Kupffer cells in the liver, the complexes continue to circulate—becoming trapped and deposited in tissue including:

  • Kidneys
  • Lungs
  • Skin
  • Joints
  • Blood vessels

The presence of immune complexes in joints or organs results in tissue damage and initiation of an inflammatory response. 

These troubling and complicated immune complex disorders may—or may not—develop into an autoimmune disease.

Immunodeficiency Diseases

Immunodeficiency diseases occur when one or more components of the immune system are disabled, or lacking.  Immune deficiencies can be inherited, produced through medical treatment, or acquired through illness, or infection.  Immune function itself declines with age.

Whatever the reason, lack of immune function makes the body seriously vulnerable to infection, and disease.

The immune system can be depressed temporarily by a viral infection.  When the effect is more lasting, causes might include:

  • Advanced cancer treatments
  • Infections like influenza, infectious mononucleosis, or measles
  • Blood transfusions, or surgery
  • Malnutrition and stress
  • Birth defects including flaws in B-cell components unable to produce antibodies
  • Birth defects that involve the thymus may cause a lack, or deficiency, of T-cells

What is Severe Combined Immunodeficiency Disease (SCID)?

Babies suffering SCID are born with no major immune function.  These extreme, rare cases require life in a contained, germ-free environment.  Some SCID patients are treated successfully through bone marrow transplants.

What About Acquired Immunodeficiency Syndrome (AIDS)?

AIDS is a devastating disorder of the immune system first recognized in 1981. The syndrome is caused by the human immunodeficiency virus (HIV).  Facts about AIDS include:

  • HIV destroys T4 cells and shelters within macrophages
  • AIDS is commonly recognized in uncommon infections and rare cancers
  • Damaging the brain and spinal cord, AIDS leaves no area of the body unravaged
  • Infections associated with AIDS are not caused by HIV, but are pathogens looking for an opportunistically vulnerable host

Research into immunodeficiency diseases grew rapidly with the discovery of HIV in human populations.  At present, there is no cure for HIV, but progress continues on life-extending medications and hopefully someday soon—a vaccine.

Cancers of the Immune System

Uncontrollable growth of any kind of cell in the human body can lead to cancer.

In the immune system, unchecked growth of white blood cells, or leukocytes, plasmas, or growths in the lymphoid organs cause:

  • Leukemias
  • Leukocytes
  • Lymphomas, like Hodgkin’s disease

Often treatable, these diseases are addressed with medication and oftentimes, irradiation.

Continuing research into the immune system will hopefully unlock reasons why immune cells grow unchecked—or why their viral nature is not suppressed by a healthy immune system.

Bone Marrow Transplants, Privileged Immunity, and Cancer

transplants, immunity

Bone Marrow Transplants

Immune cells are formed in bone marrow.  When immune function declines due to medical treatment or inherited illness, a bone marrow transplant is a possible option.

Used to treat patients with cancers of the blood, blood forming organs and lymphoid system, bone marrow transplants aim to restore stem cells destroyed by damage, disease, or defect.

Points about bone marrow transplants include:

  • Donor bone marrow is treated to remove cell types, like mature T-cells that could be dangerous to a bone marrow recipient by causing graft-versus-host disease.
  • Once implanted, transplant cells travel to bone marrow where they grow into functioning B-cells, or T-cells, which later travel to the thymus.
  • As with all tissue and organ transplant, self markers must be a very close match to avoid rejection of donor material by the recipient. 

Different forms of bone marrow transplants include:

  • Autologous:  Patients receive their own stem cells
  • Syngeneic:  Stem cells from an identical twin are transplanted
  • Allogenic:  Patients receive stem cells from a close relative—or a close cellular match

A non-profit organization, the National Marrow Donor Program (NMDP) manages a marrow registry of more than 11 million donors and umbilical cord units.

When needed, a bone marrow transplant can prove a life-saving option for those with blood cancers, or immune deficiencies.

Immunology and Transplants

Immunology plays an important role in tissue and organ transplant technology.  Without the right match, a life can be endangered, and transplant tissue lost.

More than 25 years ago, organ transplant technology was developed.  Since then, transplant lists are maintained for individuals with serious illness who may need a replacement organ.  More and more people each year designate themselves as organ donors in the event of a fatal accident.

In addition to tissue, surgeons have transplanted the heart, lungs, liver, kidney, and pancreas.

Ensuring Transplant Success

Transplant success depends on the immune system.  With a close match to self markers, and immune suppression in the recipient, an organ transplant can be successful.

Methods by which donor teams try to ensure success include:

  • Careful tissue typing, or histocompatibility testing.  Typing examines self markers on body tissues of donor and recipient.  Markers are called human leukocyte antigens (HLA).
  • Not surprisingly, the best matches are identical twins, and close relatives.
  • By transplant time, the immune system of the recipient is methodically suppressed in order to avoid rejection. Tissue recipients taking immunosuppresents have long term vulnerability to opportunistic infections.

Despite the complexity of typing, and suppressing rejection, transplants offer a new lease on life to those in need.

Privileged Immunity

Research continues into immune privilege—or areas in the body where antigens are tolerated without immune response.

An example of immune privilege exists in the womb.  In the genetic confluence that is a fetus, the child carries paternal antigens and incompatible self antigens from its mother.  While this sounds like a textbook case for immune rejection, research has shown the immune function in the uterus is dimmed, evolutionarily avoiding an immunological battle.

Research suggests the fetus produces a substance that triggers development of specialized white blood cells that mediates interaction between antibodies and antigens.  As well, a chemical produced in the uterus coats the fetal surface of the placenta, shielding it from maternal immune action.

Studies into mechanisms of immune system have located other areas, besides the womb, where this specialized privilege operates.

Immunity and Cancer

Each day, cells in your body go rogue.  Infiltrated by virus, affected by stress, cells change behavior and may grow abnormally.  On those same days, your immune system, forever filtering, makes note of the abnormality. 

Your immune system is your best defense against cancer. Cells turning cancerous experience surface change to their antigens, and the immune system normally dispatches immune cells to dispose of the altered cell.

Except when it does not.  Why?

Theories of Immune Involvement in the Development of Cancer

Cancer is uncontrolled cell growth.  Theories of the development of cancer cells include:

  • Tumors develop when components of the immune system decline or fail.
  • It is possible cancerous tumors have the ability to disguise antigens to avoid detection by the immune system.
  • Tumors may secrete a substance that suppresses or blocks immune function.

Immune-Based Cancer Research

Research continues to explore interaction between the immune system and cancerous cells in search of methodologies and processes to save lives and restore health.  Some of the methods studied include:

  • According to one study, monoclonal antibody-based cancer treatment is one of the most successful strategies for treating malignancies in the last two decades. 
  • Studies are looking at coating nanoparticles with membrane from cancer cells to develop and effectively deliver an anti-cancer vaccine.
  • Initial studies are looking at means to boost the number of T-cells in mice with diminished immune capacity.

Each study adds to, or alters, our information about the close relationship between cancer, and immune system function.  Hopefully future studies can provide game-changing techniques to battle this unrelenting enemy.

Advancements in the Understanding of the Immune System

immune advancements

The Immune System and the Nervous System

In what is not a surprising development, a growing field of study is looking at the mind-body connection, or more accurately, the mind-immune system connection.

  • Psychoneuroimmunology (PNI) is the study of the relationships between psychological processes of the brain, the nervous, and immune system.

Did you ever feel butterflies in your stomach, and then feel sick, when you were about to do something scary, or for the first time?  This immediate response is a simple example of the complicated reaction of your body to the environment in which you live.

With an explosion in alternative and complementary healthcare modalities, more people each day recognize the need to understand themselves on a cellular level.  What you eat, and what you do, has an effect on your body—and your immune system.

Mind Inside of Matter

PNI takes a holistic, instead of segmented, view of the major systems of the body.  Psychology is influenced by, and influences neurology, and the same can be said for the immune system.  We are the sum of our parts, and none is more important to human health, than another.

Points we know about PNI:

  • Stress is considered a factor in development of disease and cancer.  Lose a spouse, lose a job, get married, sell your house—all these events could affect your health.
  • Signaling between the brain, spinal cord, spinal nerves, nerves, and endocrine system affect the immune system. 
  • Release of stress hormones in a situation of chronic or acute stress dampens production and maintenance of B-cells, T-cells, NK cells, and antibody function.
  • Immune cells communicate inside and outside the immune system each day. Nerve fibers connect the thymus, spleen, lymph nodes and bone marrow.  If you are stressed—your entire body gets the message.

Integrative thinking and health practices are essential to maintaining emotional, psychological, physical and immune health.  Eat the right foods, exercise, practice relaxing modalities—your mind, and your immune system, will thank you for it.

Hybridoma Technology: What Is It?

Bioengineering took another step forward with the development of hybridoma technology.  First explored in 1975, this relatively recent science forms hybrid cell families to create monoclonal antibodies needed to diagnose, prevent, and treat disease.

By putting a B-cell together with a cancerous immune cell, a hybrid product is created that has properties of both cells under a single membrane.  The resulting hybrid is termed a monoclonal antibody. Mouse studies created foundational ground for use of hybridoma’s for human applications.

Properties of a monoclonal antibodies include:

  • These hybrids can be cloned, resulting in identical cells that secrete the same cellular antibody.
  • Monoclonal antibodies are used for research in identifying cancer types and developing appropriate treatments.
  • Some types of monoclonal antibodies are used to track or attack cancer, quantify immune cells of AIDS patients, and address potential organ transplant rejection.
  • Genetic engineering uses monoclonal antibodies to create new proteins, and develop new vaccines.
  • Hybrid antibodies are altered to behave like enzymes, and used to better control catalytic processes.

Hybridoma technology promises to open more doors to discovery in medicine, industry, and as-yet undiscovered applications.

The SCID Mouse

Severe Combined Immunodeficiency Disease (SCID) is a condition involving the total absence of any major immune function. Mouse studies prove invaluable in studying this serious genetic defect.

In the 1980s a group of scientists conducted mouse studies to study SCID. Through transplantation, and application of human immune cells in a SCID-impacted mouse, researchers created the opportunity to study a close, proximate, example of the human immune system.

This research makes it possible to study the effect of drugs, viruses, immune disorders, and other conditions in a viable mammal immune system.

Genetic Engineering

Along with epigenetics, the microbiome, immunotherapy, and antibodies, the National Institute of Health (NHI) identifies genetic engineering (GE) as a fast-developing area in immune system studies.

Well-known for its role in modified foods, GE is a major force in science and medical technology.  As a critical tool, GE is the use of tools to manipulate genetic material to pursue a variety of research aims, including:

  • Understand disease causes and develop treatments
  • Develop gene-based therapies to treat physical defect and disease
  • Modification of gene products to address or prevent illness, disease, or defect

Work toward these lofty goals is already underway as geneticists work to solve the riddle of severe combined immunodeficiency disease (SCID).   From developing a vaccine for hepatitis to the hoped-for vaccine for HIV, or cancer, GE is likely to have enormous impact on biotech in the future.

Other therapy and research projects driven by GE include studies involving:

  • Parkinson’s disease, diabetes, and hemophilia, and AIDS
  • Cancer, immunogenic tumor antigens, and suppressor genes

We owe our extraordinary immune function to a vast, networked system of organs, immune cells, and structures.  GE promises important new tools in our quest—and need—to understand that system, and help us live more comfortable, healthier lives.

The Stem Cell

Stem cells are the shape shifters of the human body, holding hope as a means to regenerate tissue, renew health, and even hold back time.

New technology continues to yield avenues of research for future stem cell studies.  Mouse and other studies suggest immune system renewal can be achieved, when purified stem cells are deployed.

From unlocking information on cell development, disease, and aging, to development of cell-based therapies, stem cell research may revolutionize the treatment of illness and genetic defects in the future.

Immunoregulation Research

Research into the functioning of the immune system continues to advance.  Along with studies of the brain, and the nervous system, human physical self-regulation is a topic that will see more studies, and more discovery—in the coming decade.

In the future, we could treat autoimmune diseases through suppression, stimulate immune cells that are underactive, and prevent rejection of tissue and organ transplants.

From our knowledge today—to new technology and processes tomorrow, immunoregulation research is a promising field of research for individuals, and our culture.

Glossary of Immune System Terms

immune system glossary
Mary Shomon

Acquired immunodeficiency syndrome (AIDS):A life-threatening disease caused by a virus and characterized by breakdown of the body's immune defenses.

Active immunity:Immunity produced by the body in response to stimulation by a disease-causing organism or a vaccine.

Agammaglobulinemia:An almost total lack of immunoglobulins, or antibodies.

Allergen:Any substance that causes an allergy.

Allergy:An inappropriate and harmful response of the immune system to normally harmless substances.

Anaphylactic shock:A life-threatening allergic reaction characterized by a swelling of body tissues including the throat, difficulty in breathing, and a sudden fall in blood pressure.

Anergy:A state of unresponsiveness, induced when the T cell's antigen receptor is stimulated, that effectively freezes T cell responses pending a "second signal" from the antigen-presenting cell (co-stimulation).

Antibody:A soluble protein molecule produced and secreted by B cells in response to an antigen, which is capable of binding to that specific antigen.

Antibody-dependent cell-mediated cytotoxicity (ADCC):An immune response in which antibody, by coating target cells, makes them vulnerable to attack by immune cells.

Antigen:Any substance that, when introduced into the body, is recognized by the immune system.

Antigen-presenting cells:B cells, cells of the monocyte lineage (including macrophages as well as dendritic cells), and various other body cells that "present" antigen in a form that T cells can recognize.

Antinuclear antibody (ANA):An autoantibody directed against a substance in the cell's nucleus.

Antiserum:Serum that contains antibodies.

Antitoxins:Antibodies that interlock with and inactivate toxins produced by certain bacteria.

Appendix:Lymphoid organ in the intestine.

Attenuated:Weakened; no longer infectious.

Autoantibody:An antibody that reacts against a person's own tissue.

Autoimmune disease:A disease that results when the immune system mistakenly attacks the body's own tissues. Rheumatoid arthritis and systemic lupus erythematosus are autoimmune diseases.

Bacterium:A microscopic organism composed of a single cell. Many but no all bacteria cause disease.

Basophil:A white blood cell that contributes to inflammatory reactions. Along with mast cells, basophils are responsible for the symptoms of allergy.

B cells:Small white blood cells crucial to the immune defenses. Also known as B lymphocytes, they are derived from bone marrow and develop into plasma cells that are the source of antibodies.

Biological response modifiers:Substances, either natural or synthesized, that boost, direct, or restore normal immune defenses. BRMs include interferons, interleukins, thymus hormones, and monoclonal antibodies.

Biotechnology:The use of living organisms or their products to make or modify a substance. Biotechnology includes recombinant DNA techniques (genetic engineering) and hybridoma technology.

Bone marrow:Soft tissue located in the cavities of the bones. The bone marrow is the source of all blood cells.

Cellular immunity:Immune protection provided by the direct action of immune cells (as distinct from soluble molecules such as antibodies).

Chromosomes:Physical structures in the cell's nucleus that house the genes. Each human cell has 23 pairs of chromosomes.

Clone:(n.)A group of genetically identical cells or organisms descended from a single common ancestor; (v.) to reproduce multiple identical copies.

Complement:A complex series of blood proteins whose action "complements" the work of antibodies. Complement destroys bacteria, produces inflammation, and regulates immune reactions.

Complement cascade:A precise sequence of events usually triggered by an antigen-antibody complex, in which each component of the complement system is activated in turn.

Constant region:That part of an antibody's structure that is characteristic for each antibody class.

Co-Stimulation:The delivery of a second signal from an antigen-presenting cell to a T cell. The second signal rescues the activated T cell from anergy, allowing it to produce the lymphokines necessary for the growth of additional T cells.

Cytokines:Powerful chemical substances secreted by cells. Cytokines include lymphokines produced by lymphocytes and monokines produced by monocytes and macrophages.

Cytotoxic T cells:A subset of T lymphocytes that can kill body cells infected by viruses or transformed by cancer.

Dendritic cells:White blood cells found in the spleen and other lymphoid organs. Dendritic cells typically use threadlike tentacles to enmesh antigen, which they present to T cells.

DNA (deoxyribonucleic acid):Nucleic acid that is found in the cell nucleus and that is the carrier of genetic information.

Enzyme:A protein, produced by living cells, that promotes the chemical processes of life without itself being altered.

Eosinophil:A white blood cell that contains granules filled with chemicals damaging to parasites, and enzymes that damp down inflammatory reactions.

Epitope:A unique shape or marker carried on an antigen's surface, which triggers a corresponding antibody response.

Fungus:Member of a class of relatively primitive vegetable organism. Fungi include mushrooms, yeasts, rusts, molds, and smuts.

Gene:A unit of genetic material (DNA) that carries the directions a cell uses to perform a specific function, such as making a given protein.

Graft-versus-host disease (GVHD):A life-threatening reaction in which transplanted immunocompetent cells attack the tissues of the recipient.

Granulocytes:White blood cells filled with granules containing potent chemicals that allow the cells to digest microorganisms, or to produce inflammatory reactions. Neutrophils, eosinophils, and basophils are examples of granulocytes.

Helper T cells:A subset of T cells that typically carry the T4 marker and are essential for turning on antibody production, activating cytotoxic T cells, and initiating many other immune responses.

Hematopoiesis:The formation and development of blood cells, usually takes place in the bone marrow.

Histocompatibility testing:A method of matching the self antigens (HLA) on the tissues of a transplant donor with those of the recipient. The closer the match, the better the chance that the transplant will take.

HIV (human immunodeficiency virus):The virus that causes AIDS.

Human leukocyte antigens (HLA):Protein in markers of self used in histocompatibility testing. Some HLA types also correlate with certain autoimmune diseases.

Humoral immunity:Immune protection provided by soluble factors such as antibodies, which circulate in the body's fluids or "humors," primarily serum and lymph.

Hybridoma:A hybrid cell created by fusing a B lymphocyte with a long-lived neoplastic plasma cell, or a T lymphocyte with a lymphoma cell. A B-cell hybridoma secretes a single specific antibody.

Hypogammaglobulinemia:Abno rmally low levels of immunoglobulins.

Idiotypes:The unique and characteristic parts of an antibody's variable region, which can themselves serve as antigens.

Immune complex:A cluster of interlocking antigens and antibodies.

Immune response:The reactions of the immune system to foreign substances.

Immunoassay:A test using antibodies to identify and quantify substances. Often the antibody is linked to a marker such as a fluorescent molecule, a radioactive molecule, or an enzyme.

Immunocompetent:Capable of developing an immune response.

Immunoglobulins:A family of large protein molecules, also known as antibodies.

Immunosuppression:Reduction of the immune responses, for instance by giving drugs to prevent transplant rejection.

Immunotoxin:A monoclonal antibody linked to a natural toxin, a toxic drug, or a radioactive substance.

Inflammatory response:Redness, warmth, swelling, pain, and loss of function produced in response to infection, as the result of increased flood flow and an influx of immune cells and secretions.

Interleukins:A major group of lymphokines and monokines.

Kupffer cells:Specialized macrophages in the liver.

LAK cells:Lymphocytes transformed in the laboratory into lymphokine-activated killer cells, which attack tumor cells.

Langerhans cells:Dendritic cells in the skin that pick up antigen and transport it to lymph nodes.

Leukocytes:All white blood cells.

Lymph:A transparent, slightly yellow fluid that carries lymphocytes, bathes the body tissues, and drains into the lymphatic vessels.

Lymphatic vessels:A bodywide network of channels, similar to the blood vessels, which transport lymph to the immune organs and into the bloodstream.

Lymph nodes:Small bean-shaped organs of the immune system, distributed widely throughout the body and linked by lymphatic vessels. Lymph nodes are garrisons of B, T, and other immune cells.

Lymphocytes:Small white blood cells produced in the lymphoid organs and paramount in the immune defenses.

Lymphoid organs:The organs of the immune system, where lymphocytes develop and congregate. They include the bone marrow, thymus, lymph nodes, spleen, and various other clusters of lymphoid tissue. The blood vessels and lymphatic vessels can also be considered lymphoid organs.

Lymphokines:Powerful chemical substances secreted by lymphocytes. These soluble molecules help direct and regulate the immune responses.

Macrophage:A large and versatile immune cell that acts as a microbe-devouring phagocyte, an antigen-presenting cell, and an important source of immune secretions.

Major histocompatibility complex (MHC):A group of genes that controls several aspects of the immune response. MHC genes code for self markers on all body cells.

Mast cell:A granule-containing cell found in tissue. The contents of mast cells, along with those of basophils, are responsible for the symptoms of allergy.

Microbes:Minute living organisms, including bacteria, viruses, fungi and protozoa.

Microorganisms:Microscopic plants or animals.

Molecule:The smallest amount of a specific chemical substance that can exist alone. (The break a molecule down into its constituent atoms is to change its character. A molecule of water, for instance, reverts to oxygen and hydrogen.)

Monoclonal antibodies:Antibodies produced by a single cell or its identical progeny, specific for a given antigen. As a tool for binding to specific protein molecules, monoclonal antibodies are invaluable in research, medicine, and industry.

Monocyte:A large phagocytic white blood cell which, when it enters tissue, develops into a macrophage.

Monokines:Powerful chemical substances secreted by monocytes and macrophages. These soluble molecules help direct and regulate the immune responses.

Natural killer (NK) cells:Large granule-filled lymphocytes that take on tumor cells and infected body cells. They are known as "natural" killers because they attack without first having to recognize specific antigens.

Neutrophil:A white blood cell that is an abundant and important phagocyte.

Nucleic acids:Large, naturally occurring molecules composed of chemical building blocks known as nucleotides. There are two kinds of nucleic acids, DNA and RNA.

OKT3:A monoclonal antibody that targets mature T cells.

Opportunistic infection:An infection in an immunosuppressed person caused by an organism that does not usually trouble people with healthy immune systems.

Opsonize:To coat an organism with antibodies or a complement protein so as to make it palatable to phagocytes.

Organism:An individual living thing.

Parasite:A plant or animal that lives, grows and feeds on or within another living organism.

Passive immunity:Immunity resulting from the transfer of antibodies or antiserum produced by another individual.

Peyer's patches:A collection of lymphoid tissues in the intestinal tract.

Phagocytes:Large white blood cells that contribute to the immune defenses by ingesting microbes or other cells and foreign particles.

Plasma cells:Large antibody-producing cells that develop from B cells.

Platelets:Granule-containing cellular fragments critical for blood clotting and sealing off wounds. Platelets also contribute to the immune response.

Polymorphs:Short for polymorphonuclear leukocytes or granulocytes.

Proteins:Organic compounds made up of amino acids. Proteins are one of the major constituents of plant and animal cells.

Protozoa:A group of one-celled animals, a few of which cause human disease (including malaria and sleeping sickness).

Rheumatoid factor:An autoantibody found in the serum of most persons with rheumatoid arthritis.

RNA (ribonucleic acid):A nucleic acid that is found in the cytoplasm and also in the nucleus of some cells. One function of RNA is to direct the synthesis of proteins.

Scavenger cells:Any of a diverse group of cells that have the capacity to engulf and destroy foreign material, dead tissues, or other cells.

SCID mouse:A laboratory animal that, lacking an enzyme necessary to fashion an immune system of its own, can be turned into a model of the human immune system when injected with human cells or tissues.

Serum:The clear liquid that separates from the blood when it is allowed to clot. This fluid retains any antibodies that were present in the whole blood.

Severe combined immunodeficiency disease (SCID):A life-threatening condition in which infants are born lacking all major immune defenses.

Spleen:A lymphoid organ in the abdominal cavity that is an important center for immune system activities.

Stem cells:Cells from which all blood cells derive. The bone marrow is rich in stem cells.

Subunit vaccine:A vaccine that uses merely one component of an infectious agent, rather than the whole, to stimulate an immune response.

Superantigens:A class of antigens, including certain bacterial toxins, that unleash a massive and damaging immune response.

Suppressor T cells:A subset of T cells that turn off antibody production and other immune responses.

T cells:Small white blood cells that orchestrate and/or directly participate in the immune defenses. Also known as T lymphocytes, they are processed in the thymus and secrete lymphokines.

Thymus:A primary lymphoid organ, high in the chest, where T lymphocytes proliferate and mature.

TIL:Tumor-infiltrating lymphocytes. These immune cells are extracted from the tumor tissue, treated in laboratory, and reinjected into the cancer patient.

Tissue typing:See histocompatibility testing.

Tolerance:A state of nonresponsiveness to a particular antigen or group of antigens.

Tonsils and adenoids:Prominent oval masses of lymphoid tissues on either side of the throat.

Toxins:Agents produced by plants and bacteria, normally very damaging to mammalian cells, that can be delivered directly to target cells by linking them to monoclonal antibodies or lymphokines.

Vaccine:A substance that contains antigenic components from an infectious organism. By stimulating an immune response (but not disease), it protects against subsequent infection by that organism.

Variable region:That part of an antibody's structure that differs from one antibody to another.

Virus:Submicroscopic microbe that causes infectious disease. Viruses can reproduce only in living cells.

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