Complement is a system of serum proteins whose main function is to defend the body against infection. Although it has a reputation as a dauntingly complex system, this results chiefly from confusion in the nomenclature, which has varied considerably with time as more components were discovered, for example as ‘component C3’ became subdivided into several components one of which is still called ‘C3’. Moreover, three different nomenclatures have been used concurrently for the proteins of part of the system. These discrepancies have been resolved and a standard nomenclature is now in