complement

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complement A group of proteins present in blood plasma and tissue fluid that aids the body's defences following an immune response; the genes encoding it form part of the major histocompatibility complex. Following an antibody–antigen reaction, complement is activated chemically and becomes bound to the antibody–antigen complex (complement fixation). It can cause lysis of certain types of bacteria, or it can render the target cell more susceptible to phagocytosis – a process called opsonization. The complement reactions attract scavenging white blood cells (phagocytes) to the area of conflict in the body.

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