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. 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. C4d is the most clinically used marker for humoral rejection. It is a degradation product of the activated complement factor C4b. C4d is typically initiated by binding of antibodies to specific target molecules...