The complement system and adaptive immunity.

The complement system covalently attaches C3d to microbial antigens which binds to CR2 on B lymphocytes, leading to a markedly enhanced adaptive immune response to that antigen. The enhancement is mediated by the cross-linking of the CR2-CD19 complex to mIg which augments the activation of several intracellular signalling pathways. Two additional receptors of the B lymphocyte, FcgammaRIIB and CD22, have opposing effects when cross-linked to mIg, the former suppressing signalling by recruiting the inositol phosphatase, SHIP, and the latter by activating the phosphotyrosine phosphatase, SHP-1. Two principles emerge from these studies: innate immunity guides the adaptive immune response, and activation of the B lymphocyte is determined by co-receptors which evaluate the biological characteristics of antigen.