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Human Body

34 Immunology discrimination of self Childhood is when you learn the most, and nowhere more so than in the thymus, the gland that sits close to the heart. It spends your prenatal months killing any lymphocyte that gets alarmed when it encounters your own cell walls, which is most of them. So, by the time you are born, your few surviving lymphocytes only react to unfamiliar forms that are deemed to be not of you. The shapes that identify cells as yours are mainly MHC proteins, and lymphocytes brandish other proteins which dock with these. If docking falters, the lymphocyte smells a rat, and sends messages that amplify far and wide to bring the white cavalry. To avoid whimsical wars being waged, several white cells agree to arouse each others martial sides before letting full hostilities commence. Your next weapons are either custom cloned granulocytes which recognise and kill certain microbes, or antibodies, bespoke proteins produced by lymphocytes in response to an antigen. Antibodies cooperate to form attack complexes, or bind to microbes while signalling eat me to a macrophage. Few intruders are hostile, and many ride along helping out, or pretend to be you in sneaky ways. Veteran white warriors remember their battles, and can fashion weapons at short notice when the progeny of old enemies return. They are trained during childhood infections, when the immune army is learning military tactics. The thymus shrinks after your first birthday, and by dotage it has all but been replaced by fat cells. So as the years roll by, the school of discrimination between self and non self gradually fades, a thymic idea of what one life is. Above an IgG antibodyC1 complex binding to a flagellated E. coli bacterium left, triggering a cascade that leads to an attack complex piercing the intruders wall. Below a happy lymphocyte samples you, while several cells cooperate to wage war. 35
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