The C3b Receptor (CR1) on Human Blood Cells

The C3b receptor (CR1) was first isolated from human erythrocyte membranes in 1979 and shown to be a large single chain polypeptide glycoprotein with a molecular weight of 205,000 daltons. CR1 isolated from erythrocyte membranes has been shown in vitro to possess cofactor activity for the I mediated cleavage of C3b to iC3b and C4b to iC4b. It also plays a role in the prevention of lysis of bystander erythrocytes by its ability to cause the decay dissociation of C4b2a3b and C3bBb formed on these cells. In addition erythrocyte CR1 in vivo is thought to play a role in the transport of opsonised immune complexes from the circulation to the reticulo enclothrlial system where they can be removed. On unstimulated phagocytic cells the primary function of CR1 is the binding of complexes opsonised with C3 and C4 degradation while on stimulated phagocytes CR1 is able to directly mediate the phagocytosis of opsonised particles. CR1 may play a role in the regulation of B lymphocyte function and on kidney podocytes CR1 may serve to prevent complement activation on the basement membrane of the glomerulus