Complement

Article By:

Rosen, Fred S. Center for Blood Research, Department of Pediatrics, Harvard Medical School, Children's Hospital Medical Center, Boston, Massachusetts.

Last reviewed:October 2020

DOI:https://doi.org/10.1036/1097-8542.152500

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• Complement, published June 2014:Download PDF Get Adobe Acrobat Reader

A group of proteins in the blood and body fluids important for humoral immunity and the generation of inflammation. Complement, consisting of more than 30 blood proteins, is a complex immunological system needed for defense against pathogens. When activated by antigen–antibody complexes or by other agents, such as proteolytic enzymes (for example, plasmin), complement kills bacteria and other microorganisms. In addition, complement activation results in the release of peptides that enhance vascular permeability, release histamine (a biologically active amine compound that functions as a major mediator of inflammation), and attract white blood cells (chemotaxis). The binding of complement to target cells also enhances the phagocytosis of these target cells (see illustration) by white blood cells. In this capacity, the complement proteins are called opsonins. The most important step in complement system function is the activation of the third component of complement (C3), which is the most abundant complement protein in blood. See also: Antibody; Antigen; Blood; Immunity; Immunology; Inflammation; Opsonin; Phagocytosis; Protein

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