The Immune System
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Definition of The Immune System
The immune system is the body's defense system.
Description of The Immune System
It is the business of the immune system to recognize "self" (the body's own cells) and "non-self" (an antigen - a virus, fungus, bacterium, or any other cell or piece of foreign tissue). To deal with antigens, the system manufactures specialized cells to recognize infiltrators and eliminate them. Among the primary components of the system are a variety of white blood cells, constituting an intelligence and communication network that helps organize the immune response. One of its most vital and noticeable components is the skin, including the mucous membranes, which not only forms a wall against intruders, but also alerts the white blood cells if the wall is breached by invading organisms (through a wound, for instance).
Other segments of the immune system reside in various organs and locales - the tonsils and adenoids, thymus, spleen, lymph nodes, appendix, certain areas of the small intestine, and bone marrow. Many mature white blood cells are highly specialized:
The T lymphocytes (T stands for thymus-derived) have various functions, including switching on various aspects of the immune response, and then (equally important) switching them off.
Another lymphocyte, the B cell (B for bone marrow), manufactures antibodies.
A larger kind of white cell, the scavengers called phagocytes, 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.
Mast cells, important in allergic reactions, release histamines into the bloodstream.
In addition, there are killer 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 cells is called "natural" because, unlike T and B cells, it does not need to be triggered by 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.
Thanks to antibodies, the immune system possesses something resembling 'memory.' Foreign antigens that invaded the body in the past may be recognized by antibodies that can stop a repeat invasion. Once you develop immunity, it may last for some years, sometimes for life.
