Our Immune System Defends Us Against Infectious Diseases

Our immune system protects us against threats. These include viruses, bacteria and parasites causing infectious diseases, from ordinary flu to full-blown malaria. The white blood cells of the defense system are produced in the marrow of our bones. The cells are carried in the blood to specialized organs, where they develop and communicate to launch immune responses against infections. Immune responses are aggressive and must be controlled. They should be activated only when the body is threatened by disease. To learn how to start or stop the immune system we must understand how microorganisms and sick cells are recognized by white blood cells.

Some white blood cells, like macrophages, destroy and eat bacteria and damaged cells. The B cells produce antibodies, which can neutralize viruses, bacteria or toxic proteins in the blood and other body fluids. The T cells can trace microorganisms that leave the body fluids to invade cells. Each T killer cell carries receptors for one type of foreign substance, "antigen". T cells can bind to infected cells and kill them. This can prevent spread of the microorganism within the body. But how can the T cells specifically identify or "recognize" the infected cells?

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