THE COMPLEMENT-SYSTEM AND ADAPTIVE IMMUNITY

The complement system covalently attaches C3d to microbial antigens wh ich binds to CR2 on B lymphocytes, leading to a markedly enhanced adap tive immune response to that antigen. The enhancement is mediated by t he cross-linking of the CR2-CD19 complex to mIg which augments the act ivation of several intracellular signalling pathways. Two additional r eceptors of the B lymphocyte, Fc gamma RIIB and CD22, have opposing ef fects when cross-linked to mIg, the former suppressing signalling by r ecruiting the inositol phosphatase, SHIP, and the latter by activating the phosphotyrosine phosphatase, SHP-1. Two principles emerge from th ese studies: innate immunity guides the adaptive immune response, and activation of the B lymphocyte is determined by co-receptors which eva luate the biological characteristics of antigen.