In the late 19th century, serum was found to contain a nonspecific heat-labile complementary principle that interacted with antibodies to induce bacteriolysis. Ehrlich and Morgan termed this factor complement.
The complement system is composed of more than 30 serum and membrane-bound proteins, all of which are needed for normal function of complement in innate and adaptive immunity. Historically, deficiencies within the complement system have been suspected when young children have had recurrent and difficult-to-control infections.
Major function of complement system is recognition and elimination of pathogens via direct killing and/or stimulation of phagocytosis. Activation of the complement system is, however, also involved in the pathogenesis of some diseases such as cancer, rheumatic diseases, Alzheimer's disease, autoimmune diseases, age-related macular degeneration and schizophrenia.
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