C4d

C4d is a stable split product remnant of classical complement activation which becomes covalently bound to endothelium and basement membrane, after induction of the classical antibody-induced pathway. As an established marker of antibody-mediated acute renal allograft rejection and its proclivity for endothelium, this component can be detected in peritubular capillaries in both chronic renal allograft rejection as well as hyperacute rejection, acute vascular rejection, acute cellular rejection, and borderline rejection. It has been shown to be a significant predictor of transplant kidney graft survival and is an aid in treating acute rejection.