Defending Aginst Disease

Man lives in a sea of microorganisms; the immune system is his license to survive.

ROBERT GOOD'S metaphor may be mixed, but it is apt. As a swimmer in an ocean of organisms, man must have a means of identifying and resisting the ones that can harm or kill him. The major mechanism that does this, and enables man to survive, is the immune system, designed by nature to quickly recognize, attack and destroy any foreign matter that enters the body. The system is complex and depends for its function on a wide variety of highly specialized substances. Its main agents are cells called lymphocytes, which are produced by the so-called "stem cells" of the bone marrow, the mushy, reddish substance that manufactures blood components. Once formed, the lymphocytes develop into two distinct types of cells, each of which plays an important role in the immune response. Those that pass through the thymus--a small organ located just under the breastbone in children (it shrinks and virtually disappears by puberty)--become Tcells, the main agents of what immunologists call "cell-mediated immunity." They are responsible for maintaining the body's biological uniqueness by rejecting foreign matter, including transplanted tissue and organs.

The other type of lymphocyte, the B-cell, undergoes differentiation, in chickens, at least, in an organ called the bursa of Fabricius. (Where that transformation takes place in man has not yet been positively determined, but it can be assumed that the human body contains an equivalent of the bursa.) B-cells are called the agents of humoral immunity because they synthesize antibodies that circulate freely in the blood. The antibodies, actually globular proteins, help the body resist disease-causing organisms. Both the B-cells and T-cells reside primarily in the body's lymphoid tissues, which are found under the arms, in the groin, behind the ear, in the abdominal cavity and other locations. From these tissues, the cells recirculate through the body and continually monitor for the presence of potential attackers.

When a foreign organism enters the body, the lymphocytes work like an internal anti-ballistic-missile system. Coming in contact with the invader, they recognize it by means of its biochemical flag, or identification marker. Every cell and microorganism is believed to carry at least one such flag on its surface; it fits, like a key in a lock, into a site on the lymphocytes. Thus lymphocytes, which know their body's own cells, recognize others as foreign and trigger an immunological alarm. -

When the alarm is sounded, the immune system swings into action, sometimes dispatching both T-and B-cells, sometimes just one variety. T-cells multiply and attack; the foreigners are soon surrounded and isolated by rings of angry lymphocytes that cause inflammation and chemically destroy the invaders. The T-cells may also call up macrophages, large scavenger cells that literally devour and digest foreign cells.

B-cells, meanwhile, are stimulated to produce antibodies, which immunologists believe can be tailor-made to interact with each of the millions of different organisms a human may encounter in his lifetime. The antibodies lock onto foreign substances, making them far more susceptible to ingestion by macrophages and other scavenger cells.

Once an antibody has locked onto an invading cell, it can interact with a series of blood proteins called "complement," which aids in destroying the invader and makes it even more attractive to scavenger cells. By one or a combination of these actions, the intruder is broken down into chemical components that are recycled by the body or excreted as waste.

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