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Scheme of the complement system. The complement system, also known as complement cascade, is a part of the humoral, innate immune system and enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inflammation, and attack the pathogen's cell membrane. [1]
Anaphylatoxins, or complement peptides, are fragments (C3a, C4a and C5a) that are produced as part of the activation of the complement system. [2] Complement components C3, C4 and C5 are large glycoproteins that have important functions in the immune response and host defense. [3]
The C1 complex (complement component 1, C1) is a protein complex involved in the complement system. It is the first component of the classical complement pathway and is composed of the subcomponents C1q, C1r and C1s. [1] [2] [3]
Complement component 3, often simply called C3, is a protein of the immune system that is found primarily in the blood. It plays a central role in the complement system of vertebrate animals and contributes to innate immunity. In humans it is encoded on chromosome 19 by a gene called C3. [5] [6]
The classical complement pathway is one of three pathways which activate the complement system, which is part of the immune system. The classical complement pathway is initiated by antigen-antibody complexes with the antibody isotypes IgG and IgM .
The C1 complement complex binds to these antibodies resulting in its activation via cross proteolysis. This activated C1 complex cleaves C4 and C2 forming a C4bC2b complex that covalently bonds to the surface of the microbe and functions as a C3 convertase, binding and cleaving C3 into C3a and C3b. Binding of a C3b molecule to the C4bC2b ...
C5a is a protein fragment released from cleavage of complement component C5 by protease C5-convertase into C5a and C5b fragments. C5b is important in late events of the complement cascade, an orderly series of reactions which coordinates several basic defense mechanisms, including formation of the membrane attack complex (MAC), one of the most basic weapons of the innate immune system, formed ...
C3a is an effector of the complement system with a range of functions including T cell activation and survival, [2] angiogenesis stimulation, [3] chemotaxis, mast cell degranulation, [4] and macrophage activation. [5]