Understanding the Immune System

1
Understanding the Immune Response

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With the job to prevent, control, or eradicate disease, the immune system plays an important role in our everyday lives. As a complex network of specialized organs and cells, the immune system defends the body by distinguishing normal cells and tissue from any substance or organism it considers foreign.

When the immune system recognizes something as a foreign agent, it will mount an immune response. These agents can be broadly defined as being either antigens or allergens.

  • An antigen can be a bacteria, fungi, virus, parasite, toxin, or foreign substance. The immune system recognizes an antigen by it characteristic features which trigger an immune response. The aim of the immune response is to neutralize the antigen.
  • An allergen, by contrast, is a harmless substance, such as cat dander or ragweed pollen, that the body regards as an antigen. When this happens, the immune system will trigger a response we refer to as an allergic reaction.

For reasons not yet fully understood, the immune system will sometimes misidentify its own cells as foreign and mount an immune response. We refer to this as an autoimmune disease. Examples include psoriasis, rheumatoid arthritis, lupus, or type 1 diabetes.

2
The Anatomy of the Immune System

Red blood cells inside a blood vessel.
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The immune system is populated by a variety of organs, glands, and tissue that support your growth and development. These include:

  • The bone marrow is where all blood and immune cells are produced.
  • The thymus gland, located behind the breastbone is involved in the maturation of certain defensive blood cells.
  • Lymph nodes, clustered throughout the body, house a variety of immune cells needed to initiate a successful immune response.
  • The spleen contains lymphoid tissue which processes and renews blood and immune cells.
  • The lymphatic system is a highway between tissues and organs that carry lymph, a colorless fluid filled with white blood cells.

These organs are also key players in the production of lymphocytes, the white blood cells which act as first responders whenever you are injured or ill.

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 maturation. Once mature, B-cells and T-cells use the bloodstream and lymphatic system to travel continuously throughout the body.

3
Types of Immune Response

White Blood Cells
Lymphocytes (white blood cells). Credit: Henrik Jonsson/E+/Getty Images

In the presence of any disease-causing agent (pathogen), the immune system will trigger not one but two different immune responses

  • The innate immune response is considered the first-line attack to any general threat, such as a virus or bacteria. It is innate because it is always there, is always the same, and always uses the same defensive cells.
  • The adaptive immune response is one in which the immune system, upon recognizing the pathogen, creates specific cells to target and neutralize that pathogen. As such, the immune system adapts to each new pathogen.

The adaptive response relies upon both B-cells and T-cells.The B-cells work by recognizing an antigen and secreting substances called antibodies which "tag" the pathogen. The T-cells then follow up by targeting the "tagged" pathogen for destruction.

A subset of B-cells and T-cells are called memory B-cells and T-cells. These serve as immune sentries, "remembering" antigens and triggering a response should the antigen ever reappear.

4
Coordinating the Immune Response

Cytokines in an immune reaction.
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Communication within the immune system is directed in large part by chemical messages. These chemicals, called cytokines, are produced by a broad range of immune cells in response to the behaviors of the cells around them.

When released, cytokines trigger other immune cells to act or not to act. By doing so, they not only direct cell traffic and behavior, they regulate the growth and responsiveness of specific cell populations (including defensive blood cells and those involved with tissue repair).

Cytokines are similar in many ways to hormones. But, unlike those cell-signalling molecules, cytokines are involved in modulating the immune response. Hormones, by contrast, primarily regulate physiology and behavior.

Cytokines are important in health and disease, responding to infection, inflammation, trauma, sepsis, cancer, and even ​stages of reproduction.

5
The Role of Antibodies

Immunoglobulin G Antibody
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An antibody, also known as an immunoglobin, is a Y-shaped protein secreted by B-cells which have the ability to identify pathogens. The two tips of the "Y" are able to latch onto either the pathogen or infected cell and mark it for neutralization in one of three ways:

  • Preventing the pathogen from entering a healthy cell
  • Signaling other proteins to surround and devour the invader in a process called phagocytosis
  • Killing the pathogen itself

Antibodies are passed from the mother to child through a process called passive immunization. Upon birth, the child will begin to independently produce antibodies, either in response to a specific antigen (adaptive immunity) or as part of the body's natural immune response (innate immunity).

Humans are capable of producing over ten billion different types of antibodies, each targeted to a specific antigen. The antigen-binding site on the antibody, called the paratope, locks on to the complementary site on the antigen called the epitope. The high variability of the paratope allows the immune system to recognize an expansive range of antigens.

6
Understanding Allergy

Woman sneezing in tall grass.
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An allergy is caused when a person’s immune system reacts to substances that are harmless to others. We refer to these substances as allergens. While we tend to associate allergy with hay fever and pollen, an allergy can be triggered by any number of allergens including medications, foods, toxins, latex, metal, and even sun exposure.

Allergic reactions occur when your body produces antibodies, specifically immunoglobulin E (IgE), in response to a substance it deems harmful. The antibody then binds to the allergen and to either one of two white blood cells (mast cells that reside in tissue or basophils that circulated freely in the blood), triggering the release of inflammatory substances called histamines. This hyperreactive response can manifest with:

  • Respiratory symptoms such as sneezing, itching, runny nose, redness of the eyes, shortness of breath, and wheezing, often the result of airborne irritants
  • Gastrointestinal symptoms such as abdominal pain, bloating, vomiting, and diarrhea, typically related to a food allergy
  • Dermatological symptoms such as rash, hives, fever, and itchiness, caused by everything from medications and insect bites to contact with organic or inorganic substances

In certain instances, a person may experience a potentially life-threatening, all-body allergic reaction known as anaphylaxis. Symptoms include severe hives, facial swelling, respiratory distress, rapid or slow heart rate, dizziness, fainting, confusion, and shock.

Mild allergies are typically treated with antihistamines, while more serious reactions may require an injection of epinephrine.

7
Causes of Autoimmune Disease

vitiligo and thyroid disease
Vitiligo, loss of skin color, is commonly associated with autoimmune disease. Axel Bueckert /EyeEm/Getty Images

At its very heart, an autoimmune disease is the reflection of an immune system run amok, attacking normal cells and tissues it considers harmful. It's a condition we still don’t fully understand, but research suggests that numerous factors play a part (including genetics, viruses, and toxic exposure).

When the immune system malfunctions, it will release defensive lymphocytes and so-called autoantibodies which target cells in different parts of the body. This inappropriate response, which is referred to as an autoimmune reaction, can cause inflammation and tissue damage.

Autoimmune disease is not uncommon. There are over 80 known forms of the disease with symptoms ranging from mild to severe. Some of the more common include:

Treatment varies by disorder but may involve the use of corticosteroids, immune-suppressing medications, anti-cancer drugs, and plasmapheresis (plasma dialysis).

8
Understanding Immunity and Vaccines

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Vaccines are substances, organic or man-made, that are introduced into the body to trigger an immune response. The aim of the vaccine is to either prevent a disease (prophylactic vaccine), control a disease (therapeutic vaccine), or eradicate a disease (sterilizing vaccine).

Vaccines are used to fill gaps in a person’s immunity, either because a person has not been yet exposed to a pathogen (such as an annual strain of flu) or the pathogen poses a serious health threat that the immune system cannot fully control (such as the herpes zoster virus which causes shingles).

Among the different approaches to vaccine design:

  • Live attenuated vaccines are made with live, disabled virus (and sometimes bacteria) which cannot cause harm but nevertheless triggers an immune response. Measles, mumps, chicken pox, and polio are just some of the examples of live vaccines.
  • Inactivated vaccines use "killed" viruses, bacteria, or other pathogens to spur an immune response. The flu, hepatitis A, and rabies are some examples of inactivated vaccines.
  • Subunit vaccines use only a fragment of a pathogen to trigger the immune response. Both hepatitis B and human papillomavirus (HPV) are examples of subunit vaccines.
  • Toxoid vaccines are made from inactivated toxic compounds that are harmless to the body but still trigger an immune response. Vaccines for tetanus and diphtheria are produced in this way.
  • DNA vaccines are those in which modified DNA is inserted into a vector (such as a deactivated virus or bacteria). The vector is then injected into the body where it attaches to target cells and "reprograms" them to produce specific antibodies.

Source:

Rich, R.; Fleischer, T.; Shearer, W.; et al. (2012) Clinical Immunology (4th Edition). New York: Elsevier Science.

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