C4d is a split product of the complement C4, its deposition serves as sensitive and specific marker for antibody mediated rejection (AMR) of the transplanted organs.
AMR is one of the most important adverse events after organ transplantation and if untreated, leads to significant graft loss. During AMR, C4d binds covalently to the target structures, which makes it easy to detect with corresponding antibodies.
Currently all renal transplant biopsies and biopsies from other organs as well, are required to stain with C4d antibody.