Composed of roughly two trillion cells, the human body's immune system is a complex array of cells and molecules that collectively provide a formidable defense against a world full of hostile microbial invaders.

We are exposed continuously to germs from a variety of sources. Most of these germs pose no threat. The cells of the immune system constantly monitor conditions to maintain the status quo; but, occasionally, changing conditions allow certain germs to gain the advantage. For example, organisms that cause pneumonia may be present in your lungs at this very moment, but you are not sick because the normal balance is being maintained; if your immune defenses are weakened, the balance will shift and these otherwise harmless organisms will rapidly overpower your immune system, leading to disease.

The white blood cells are smart enough to identify invaders in the body, whether they are viruses, bacteria, toxins, or parasites. Whether or not they have seen the invaders before, immune cells can recognize foreign substances - also called antigens. When a germ penetrates the front-line defenses of the skin or mucous membranes and invades the tissue, the body responds with a process known as inflammation. White blood cells are ushered to the site where they gobble up foreign cells and substances. The same white blood cells also may release special chemicals to help destroy the invader.

When a germ invades the body, a Y-shaped antibody molecule may be released to interact with the germ. The ends of the arms of each antibody are virtually unique and make it specially adapted to a particular germ or toxin (the antigen). The antibody binds to the antigen by its unique end - like a key to a lock. Once bound, antibodies act like beacons to help immune cells recognize the invader. Now, the immune cells can recognize and gobble up the antigens at a faster rate.

Because the white blood cells can remember a germ once they have seen it, the next time you are exposed to that particular germ, you will not get sick. This capacity to recall also means that vaccines can be developed to prevent infections. Vaccines give the immune system an opportunity to become familiar with a disease organism, such as polio virus, that has been rendered harmless and allows a rapid response to any subsequent exposure, thus preventing the disease.

The existence of diseases such as AIDS and cancer, however, illustrates that the immune system can be outsmarted. In the case of AIDS, the HIV virus can hide inside the body's immune cells as it systematically destroys the immune system. In the case of cancer, tumor cells can produce factors that actually coax certain immune cells into helping the tumor grow.

By studying AIDS and cancer, scientists have uncovered a great deal about how the immune system works. Many of these principles have been used to develop experimental therapies. For certain rare diseases, factors naturally produced by the immune system are now made synthetically and marketed for therapy. A basic understanding of normal immune functions will allow researchers to improve existing therapies, and develop new approaches, for many diseases.

Written by Klaus Elgert. Dr. Elgert is author of the introductory immunology textbook, Immunology: Understanding the Immune System, published by John Wiley & Sons, 1996.)

Also see related Virginia Tech Research Magazine Article | Klaus Elgert's Lab page