The complement factor 3 (C3) gene, with 41 exons spanning 41kb on genomic DNA, is mapped to human chromosome 19p13.3. It is the third component of complement and is characterized with α and β chains. It is one of the most abundant complement protein in the serum.
Complement factor 3 (C3) plays an essential role in the activation of the complement system. C3-convertase (C4b2a) cleaves C3 complement into C3a and C3b. C3b can bind covalently, through its reactive thioester, to cell surface carbohydrates or immune complexes and involve in the self-activation loop of complement activation via the alternate pathway. In addition, C3b also interacts with C3 convertases (C4b2a or C3bBb) to form the C5 convertase, which cleaves complement factor 5 into C5a and C5b. Derived from proteolytic degradation of complement C3, C3a (anaphylatoxin) facilitates the recruitment of inflammatory cells. C3a stimulates smooth muscle contraction, enhances vascular permeability and causes histamine release from mast cells and basophilic leukocytes. Mutation in C3 gene leads to the development of age-related macular degeneration (AMD).
