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How does the immune system work?

The immune system has two main components, white blood cells and antibodies, both of which circulate in the blood. When an antigen (antibody generator), such as a harmful bacterium, virus, fungus, parasite, or other foreign substance invades the body, a specific antibody is generated to attack the antigen.

Source: McAleer, Neil. The Body Almanac, p. 143.

The antibody is produced by beta-lymphocytes (B-cells) in the spleen or lymph nodes. An antibody may either destroy the antigen directly or it may "label" it, instructing a white blood cell (called a microphage, or scavenger cell) to swallow up the antigen.

Once a person has been exposed to an antigen, subsequent exposures to the same antigen will produce an immune system reaction. The necessary antibodies will be produced more rapidly and in larger amounts than they were on initial exposure.

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