In physiology, a complex system of at least 20 proteins (complement components) in normal blood serum. The binding of one component to an antigen-antibody complex begins a chemical chain reaction important in many immunological processes, including breakdown of foreign and infected cells, ingestion of foreign particles and cell debris, and inflammation of surrounding tissue. Complement components and antibodies are the substances in human serum responsible for killing bacteria.
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In many different fields, the complement of X is something that together with X makes a complete whole, something that supplies what X lacks.
Complement may refer to: