How does our immune system work against a foreign body?

The immune system is the body's protection against invasion of antigens such as bacteria, viruses, pollen, additives, certain foods, and others. When a person is first introduced to an antigen, such as a flu virus, the immune system makes an antibody, or protective chemical, to fight off the invader. The next time the person is exposed to the same flu virus, the immune system remembers, recognizes the invader, and releases the specific antibodies that react with the virus and destroy it. Incredibly, the immune system can produce antibodies with the identical shape of the antigen they must bind with point for point in order to eliminate the antigen, allowing the person to recover. Antibodies are made by white blood cells called lymphocytes, which originate in bone marrow and circulate through the bloodstream, patrolling all areas of the body to identify and protect against foreign invaders. When the lymph nodes in the body swell, it's actually because they're filling up with these cells which will attack the invader.

Today, through state-of-the-art biotechnology, scientists can now identify two basic types of lymphocytes called B cells and T cells. The B cells produce antibodies that neutralize certain components of invaders (such as in chicken pox or mumps). The T cells act more directly to kill foreign invaders. Some are cytotoxic cells, which punch holes in the antigen. Others are T-helper cells, which enhance the role of other immune system cells and can activate the macrophages, which eat every foreign invader in their path, almost like a Pac Man gobbling up debris. Secretions from the T-helper cells also activate natural killer (NK) cells, which play an important but not yet fully understood role in fighting viral infections. Finally, there are T-suppressor cells which turn off the immune system response once the invader is completely destroyed.

The immune system, made up of many subsets of B and T cells, is a very carefully regulated system. Besides the interaction between lymphocytes, macrophages, and antibodies, the immune system also recognizes self or nonself. In other words, it can tell the difference between your own liver and a transplanted one. It will know that the new liver is not the one that should be there, genetically, and it will reject it unless the immune system is toned down through such immunosuppressant drugs as cyclosporine.