How does the immune system fight disease?
Because war is a handy metaphor for the human body's reaction to disease, science writers like to describe the immune system in militaristic termsthe body's department of defense. But unlike the Armed Forces, the immune system has no headquarters or commander-in-chief. And its operations are usually swifter and more efficient than any army's could be.
Rather than "making war," your immune system is really more like an immigration service: a highly differentiated cellular bureaucracy that supervises your biological commerce with the outside world, sorts through billions of pieces of information about incoming materials, and takes routine action as required. Only occasionally does it declare an emergency.
The immune system's basic task is to recognize "self" (the body's own cells) and "nonself" (an antigena virus, fungus, bacterium, or any piece of foreign tissue, as well as some toxins.) To deal with nonself or antigens, the system manufactures specialized cellswhite blood cellsto recognize infiltrators and eliminate them. We all come into the world with some innate immunity. As we interact with our environment, the immune system becomes more adept at protecting us. This is called acquired immunity.
What are the parts of the system?
Among the primary components of the immune system are a variety of white blood cells. These constitute a communications network that helps organize the immune response.
Most people are surprised to learn that the skin, including the mucous membranes, is among the most vital components of immunity. The skin not only forms a wall against intruders, but actually alerts the white blood cells if the wall is breached by invading organisms (through a wound, for instance.) The protection afforded by the intact skin is why it's nearly impossible to catch a disease from a toilet seat, for example.
Most infectious agents get inside the body when we inhale them or swallow them; a few can enter through the genitals. They make their way into the blood and move rapidly through the body. The immune system has its own circulatory system called lymphatic vessels, which allow white blood cells to catch intruders. Other important parts of the immune system include the tonsils and adenoids, thymus, spleen, lymph nodes, appendix, certain areas of the small intestine and bone marrow.
What do the white cells do?
Many mature white blood cells are highly specialized. The so-called T lymphocytes (T stands for thymus-derived) have various functions, among them switching on various aspects of the immune response, and then (equally important) switching them off. Another lymphocyte, the B cell, manufactures antibodies. A larger kind of white cell, the scavenger, called the phagocyte (most notably the macrophage,) eats up all sorts of debris in tissue and the bloodstream and alerts certain T cells to the presence of antigens.
In addition, there are killer, suppressor, and helper T cells. Killer T cells, stimulated by helper T cells, zero in on cells infected by antigens, or turn against the body's own cells when, as in the case of cancer, they begin to proliferate abnormally. Another class of lymphocyte killer cell is called "natural" because, unlike T and B cells, it doesn't need to recognize a specific antigen. Most healthy cells are of no interest to natural killer cells, but cancer cells and cells invaded by viruses may be vulnerable to their search-and-destroy missions.