Immune System & Defenses – Part 2 (end)

Specific immune defenses

The specific immune response is activated when microorganisms survive or get past the non-specific defenses. Two types of specific defenses destroy microorganisms in the human body: the cell-mediated response and the antibody response. The cell-mediated response attacks cells which have been infected by viruses. The antibody response attacks both "free" viruses that haven't yet penetrated cells and bacteria. Most bacteria do not infect cells, although some do, such as the Mycobacteria that cause tuberculosis . The specific immune response depends on the ability of the immune lymphocytes to identify the invader and create immune cells that specifically mark the invader for destruction. Bone marrow produces an amazing array of lymphocytes, each of which is capable of recognizing one specific molecular shape called an antigen.

Two kinds of lymphocytes operate in the specific immune response: T lymphocytes and B lymphocytes, (T lymphocytes are made in the thymus gland, while B lymphocytes are made in bone marrow). B and T lymphocytes are individually configured to attack a specific antigen. For example, the blood and lymph of humans have T-cell lymphocytes that specifically target the chicken pox virus, T-cell lymphocytes that target the diphtheria virus, and so on. When T-cell lymphocytes specific for the chicken pox virus encounter a body cell infected with this virus, the T-cell multiplies rapidly and destroys the invading virus.

After the invader has been neutralized, some T cells remain behind. These cells, called memory cells, impart immunity to future attacks by the virus. Once a person has had chicken pox , memory cells quickly stave off subsequent infections. This secondary immune response, involving memory cells, is much faster than the primary immune response.

When a human is immunized against a disease, the vaccination injects whole or parts of killed viruses or bacteria into the bloodstream, prompting memory cells to be made without a person developing the disease.

Helper T cells are a subset of T-cell lymphocytes present in large numbers in the blood and lymphatic system, lymph nodes, and Peyer's patches. When one of the body's macrophage cells ingests a foreign invader, it displays the antigen on its membrane surface. These antigen-displaying-macrophages, or APCs, are the immune system's distress signal. When a helper T cell encounters an APC, it immediately binds to the antigen on the macrophage. This binding unleashes several powerful chemicals called cytokines. Some cytokines stimulate the growth and division of T cells, while others play a role in the fever response. Still another cytokine, called interleukin II, stimulates the division of cytotoxic T cells, key components of the cell-mediated response. The binding also "turns on" the antibody response. Any disease, such as HIV, that destroys helper T cells destroys the immune system.

Antibodies are made when a B cell specific for the invading antigen is stimulated to divide. The dividing B cells, called plasma cells, secrete antibodies composed of a special type of protein called immunoglobin (Ig).

T cells

T-cell lymphocytes are the primary players in the cell-mediated response. When an antigen-specific helper T cell is activated, the cell multiplies. The cells produced from this division are called cytotoxic T cells. Cytotoxic T cells target and kill cells that have been infected with a specific microorganism. After the infection has subsided, a few memory T cells persist, so conferring immunity.

Chemical signals activate the immune response; likewise, chemical signals must turn it off. When all the invading microorganisms have been neutralized, special T cells (called suppressor T cells) release cytokines that deactivate the cytotoxic T cells and the plasma cells, and the cells of the body return to normal functioning.

Immune system disorders

Sudden Infant Death Syndrome

In 1994, researchers reported in the medical journal The Lancet that abnormal immune response in the respiratory system may contribute to sudden infant death syndrome (SIDS). Two to three times as many T-lymphocytes were found in lungs of children who died from SIDS that in those who died from other causes. In addition, the number of B-lymphocytes appears to be higher in SIDS infants than in others.

The World Resources Institute in Washington, DC, issued a report in 1996 linking the increased exposure to chemical pesticides in the environment and immune system disorders. Developing nations are at the greatest risk, since they often do not regulate pesticide use. The Institute cited the former Soviet republic of Moldova, where, from 1960 to the late 1980s, pesticides were used in concentrations nearly 20 times the average used elsewhere in the world.

Eighty percent of children known to have been exposed to the pesticides appear to have irregularities in their immune systems. Because the interpretation of the study results is difficult, more research is needed.

Further Reading

For Your Information

Books

• Almonte, Paul. The Immune System. Crestwood House; Maxwell Macmillan Canada; Macmillan International, 1991.
• Cook, Allan R. Immune System Disorders Sourcebook: Basic Information for the Layperson. Detroit, MI: Omnigraphics, 1996.
• Edelson, Edward. The Immune System. New York: Chelsea House, 1989.
• Schindler, Lydia Woods. The Immune System: How It Works. Bethesda, MD: U. S. National Institutes of Health, 1993.

Periodicals

• Engelhard, Victor H. "How Cells Process Antigens." Scientific American 271, August 1994, p. 54.
• Kedzierski, Marie. "Vaccines and Immunisation (sic)." New Scientist 133, February 8, 1992, p. S1.
• Kisielow, Pavelrod. "Self-Nonself Discrimination by T Cells." Science 248, June 15, 1990, p. 1369.
• Miller, Jacques. "The Thymus: Maestro of the Immune System." BioEssays 16, July 1994, p. 509.
• Radesky, Peter. "Of Parasites and Pollens." Discover 14, September 1993, p. 54.
• "Special Issue: Life, Death, and the Immune System." Scientific American 269, September 1993.
• Strange, Carolyn. "Rethinking Immunity." BioScience 45, November 1995, pp. 663+.
• Travis, John. "Tracing the Immune System's Evolutionary History." Science 261, July 9, 1993, p. 164.

Gale Encyclopedia of Childhood & Adolescence. Gale Research, 1998.