The monoclonal antibody 16D2 recognizes mouse complement factor C4, formerly known as Gg protein, which consists of an alpha-, beta-, and gamma-chain. The classical pathway of complement and the Mannose binding lectin activation pathway converge at C4. C1s, MASP-1 and MASP-2 cleave C4 resulting in the formation of C4a and C4b. Subsequently, C4b can be cleaved to C4c and C4d by other serum enzymes. The monoclonal antibody 16D2 reacts with intact C4, C4b and C4d.
C4 is an acute phase protein that is produced by hepatocytes, monocytes and intestinal epithelial cells and can be used- in experimental animals as a marker for activation of the classical complement pathway. Recent studies have demonstrated an association between graft rejection and C4d deposition in a mouse model for cardiac transplantation.