Complement component C4 is an essential component of humoral immune response. C4 plays a central role in the activation of the classical pathway of the complement system. It is processed by activated C1 which removes from the alpha chain the C4a anaphylatoxin. The remaining alpha chain fragment C4b is the major activation product and is an essential subunit of the C3 convertase (C4b2a) and the C5 convertase (C3bC4b2a) enzymes of the classical complement pathway. Each C4 contains beta chain, alpha chain,C4a anaphyltoxin, C4b and gamma chain.