Complement component 4 is a protein involved in the complement system. It is cleaved into proteins 4a and 4b. C4a is an anaphylatoxin, C4b forms part of C3-convertase, in conjunction with 2a, C4b can bind CR1, and C4d is the final proteolytic remnant of deposited C4b on endothelium, remains covalently attached to endothelium for little more than a week and easily detectable by antibody staining. Synonyms: Monoclonal Complement C4 antibody, Anti-Complement C4 antibody, C4 antibody.