CD35 (CR1) is a type 1 membrane glycoprotein participating in clearance of immune complexes, regulation of autologous complement activation on cell surfaces, and complement-dependent cellular cytotoxicity mediated by leukocytes. CD35 inhibits complement activation by two distinct mechanisms. CD35 is a cofactor for factor I cleavage of C3b, iC3b and C4b. Furthermore CD35 mediates decay-dissociation of C3 and C5 convertases. CD35 is found on erythrocytes, neutrophils, monocytes and dendritic cells.There are four allotypic variants of CD35 (CR1) with different molecular masses: 190 kDa (CR-1A), 220 kDa (CR1-B), 160 kDa (CR1-C) and 250 kDa (CR1-D).The monoclonal antibody 31R binds a single epitope of CR1.