The Biomedica C4d antibody is the most cited (>100 publications) and reliable tool to identify humoral graft rejection in paraffin embedded tissues and can be used for indirect immunoflourescence of frozen sections. Target Information: C4d is the degradation product of the activated complement factor C4, a component of the classical complement cascade, which is typically initiated by binding of antibodies to specific target molecules.

Detection of C4d is regarded as an indirect sign, a "footprint" of an antibody response against the allograft. The majority of publications describe C4d as an important marker in kidney transplantation but also in heart, liver, and other transplants.