Understanding White Blood Cells and Types of Immunity

Innate Immunity and Cell-Mediated and Humoral Acquired Immunity

What is the difference between innate and acquired immunity and what role do white blood cells play?. Istockphoto.com/Stock Photo©selvanegra

White Blood Cells and Immunity - Introduction

Our immune system, or our immunity, is a mechanism that protects our bodies from harmful substances, foreign microorganisms, and even cancer. While originally our immune system was studied in terms of its role in preventing infections, our understanding has evolved and led to a broader view of blood and marrow stem cell transplantation, blood transfusion, cancer, and genetics.

Our white blood cells (WBCs) are a part of our immune system and play a prolific role in blood and marrow cancers. In fact, the word leukemia means “white blood,” as it is related to an overproduction of white blood cells.

Functions of the Immune System

The immune system has four main functions:

  • Protection against foreign microorganisms such as bacteria, viruses, parasites, and fungi
  • Homeostasis (body equilibrium) through the removal of worn out or dead cells
  • Surveillance and removal of mutant cells
  • Regulation through increasing and suppressing immune response

The immune system is capable of carrying out these functions primarily through its ability to tell the difference between self (part of the body) and non-self (invading organisms like bacteria, fungus, and viruses, or toxins) cells. It determines this by the antigens, or proteins, on the cell surface. Cells with antigens that the immune system recognizes as the self are left alone, while a non-self antigen cell will deploy our immunity, which will respond by recruiting, activating and mobilizing the appropriate white blood cells to the location of the threat.

  One of the problems with cancer cells is that they have found ways to disguise themselves to look like self.

Types of Immunity

The two basic types of immunity are innate and acquired immunity. Some of our white blood cells play a role in innate immunity, others in acquired immunity, while some are involved in both.

Innate Immunity

Innate immunity is the first-line, non-specific response to any breach of our bodies. We are born with innate immunity. Innate immunity is carried out by four mechanisms: mechanical barriers, chemical barriers, fever, and phagocytosis or inflammation.

  • Mechanical barriers include the skin and membranes that line our mouths, nose, airways, urinary tracts, and gastrointestinal organs. When these are intact, they provide a physical barrier against the entry of toxins and harmful organisms.
  • Chemical barriers include sweat, tears, saliva, stomach acids, mucus, and other fluids secreted by the body. These prevent foreign invaders by making the environment inhospitable to them. For example, chemical barriers may make a tissue too acidic or sticky and thus deter the organism or toxin from attaching and inhabiting the body.
  • Fever helps the body defend against bacteria and viruses that are sensitive to extremes in temperature. These invaders excrete substances that trigger the body into increasing its temperature. These types of organisms cannot tolerate the elevated temperatures for an extended period of time.  (This is why it's now recommended to leave a low-grade fever alone with colds and the flu - unless you are uncomfortable - since it is one of the ways that our bodies fight those infections.)
  • Inflammation occurs when the mechanical and chemical barriers to foreign invaders have failed. The response is very fast, but also very short-lived. This is a non-specific response, that is, your body does not have to have been exposed to this invader before to respond to it. Inflammation is initiated and controlled by phagocytic WBCs, such as polymorphonuclear leukocytes or “granulocytes”-- neutrophils, monocytes, macrophages, eosinophils, basophils, and natural killer cells Neutrophils attack organisms such as bacteria and fungi, and may leave the blood for the tissues during an inflammatory response. Basophils initiate an inflammatory response to environmental antigens, whereas eosinophils defend the body against parasites. Natural killer cells use potent chemicals to kill infected cells on contact.  Macrophages act as scavenger cells in the tissues, while monocytes perform their role in the bloodstream. Once a macrophage digests a foreign invader, it presents details about that organism to T-lymphocytes and helps to recruit the acquired immune system.

Acquired Immunity

Acquired immunity, also called adaptive immunity, is a learned immune response to a specific foreign invader. Once the body becomes exposed to a foreign antigen, acquired immunity kicks in and remembers that information long-term. Many years later, when our immune system sees that same antigen again, it is already prepared for it and can launch a rapid attack. The two main mechanisms for this type of immunity are cell-mediated immunity and humoral immunity, which are both executed by lymphocytes.

Lymphocytes make up about a third of the WBCs in our bodies. Lymphocytes are small cells that can circulate in the blood, but are also able to exist in tissues, essentially roaming freely in the body looking for work. The subtypes of lymphocytes are T lymphocytes or T-cells, (which play a role in both cell- mediated and humoral immunity) and B lymphocytes or B-cells. Some B-lymphocytes become plasma cells, which in response to a particular antigen can remember an invader at a future exposure and produce antibodies to that specific antigen.

Cell-Mediated Immunity (CMI)

Cell-mediated immunity uses T-lymphocytes as its main weapon, although interaction between T-lymphocytes and B-lymphocytes often occurs. After a foreign invader is digested by a macrophage, it presents details about the antigens on the surface of that microorganism to T- lymphocytes.

One type of T-lymphocyte, the helper T-cell, will bring that information to other T- lymphocytes (so they will recognize the invader), natural killer cells (who will seek out and kill the organism), and B-lymphocytes (who initiates the humoral immune response).

Another type of T-lymphocyte, the cytoxic T-cell, uses a more direct approach and kills cells that it recognizes as non-self or potentially harmful.

Humoral immunity

Humoral immunity involves the production of antibodies. Antibodies, or immunoglobulins, are proteins produced by B-lymphocyte plasma cells in response to recognition of a specific foreign antigen. Antibodies can prevent viruses from entering healthy cells, neutralize the invader’s toxins, or break down the microorganism and leave them for the scavenger phagocytic cells to get rid of.

Working Together

Immunity is a very complex process that requires interaction between all the mechanisms to be most effective. Phagocytic WBCs, such as macrophages and natural killer cells from our innate immunity, help to make cell-mediated and humoral immunity function properly. However, our innate immune system is only effective in the short-term, and needs our acquired immunity for continued protection.

Sources:

Bonilla, F. The humoral immune response. UpToDate. Updated 03/23/15. http://www.uptodate.com/contents/the-humoral-immune-response

Johnston, R. An overview of the innate immune system. UpToDate. Updated 11/02/15. http://www.uptodate.com/contents/an-overview-of-the-innate-immune-system

Otto, S. Protective Mechanisms. in Otto, S. ed (2001) Oncology Nursing 4th ed. Mosby:St. Louis. (pp. 917-948).

Williams, L. “Comprehensive Review of Hematopoiesis and Immunology: Implications for Hematopoietic Stem Cell Transplant Recipients” in Ezzone,S. (2004) Hematopoietic Stem Cell Transplantation: A Manual for Nursing Practice. Oncology Nursing Society. Pittsburg, PA (pp.1-13).

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