The complement component 3 (c3) is a protein factor, which is part of the complement system, a cascade of serum proteins that play a role in the development of inflammatory reactions. It plays a major part in both the classical and alternative complement pathways. The C3 convertase cleaves C3 into C3a and C3b; both of which play important roles in the cascade. C3a induces vasodilation, increases capillary permeability, promotes phagocyte chemotaxis, and most importantly, is an anaphylatoxin, which acts as a chemotactic attractant for the induction of neutrophilic release of antimicrobial proteases and oxygen radicals. C3b, on the other hand, acts as an opsonin for antigen-antibody complexes, and forms immune aggregates.

C3 levels in the serum and other body fluids are used in the identification of several disease states. Increased C3 levels are seen in conditions such as cancer, rheumatoid arthritis, and ulcerative colitis, whereas reduced levels are observed in systemic lupus erythematosus, bacterial infections, cirrhosis, hepatitis, and rejection of kidney transplants.

The C3 gene is a 43 Kb stretch of DNA located on chromosome 19p. This gene, which consists of 41 exons. It codes for a C3 precursor (pro-C3), which weighs approximately 180KDa, and consists of 1663 amino acids. This pro-C3 protein has been shown to have significant sequence homology to alpha 2-microglobulin and C4. The processing of this C3 precursor involves the removal of four Arginine residues, producing two chains, of approximately 110 and 75 KDa, bound by a disulfide bond.