C3a (complement)

C3a is one of the proteins formed by the cleavage of complement component 3; the other is C3b. C3a is a 77 residue anaphylatoxin that binds to the C3a receptor (C3aR), a class A G protein-coupled receptor. It plays a large role in the immune response.

C3a molecules induce responses through the GPCR C3a receptor. Like other anaphylatoxins, C3a is regulated by cleavage of its carboxy-terminal arginine, which results in a molecule with lowered inflammatory function (C3a desarginine).

C3a is an effector of the complement system with a range of functions including T cell activation and survival, angiogenesis stimulation, chemotaxis, mast cell degranulation, and macrophage activation. It has been shown to have both proinflammatory and anti-inflammatory responses, its activity able to counteract the proinflammatory effects of C5a.

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