Description: Complement 4d (C4d) is the most clinically used marker for humoral rejection. It is a degradation product of the activated complement factor C4b.
Complement 4d is typically initiated by binding of antibodies to specific target molecules. Following activation and degradation of the C4 molecule, thio ester groups are exposed, which allow transient, covalent binding of the degradation product Complement 4d to endothelial cell s urfaces and extracellular matrix components of vascular basement membranes near the sites of C4 activation. Complement 4d is also found in intracytoplasmic vacuoles of endothelial cells. Covalent binding renders C4d a stable molecule that can easily be detected by immunohistochemistry.