Complement's participation in acquired immunity

The preliminary evidence for the in- volvement of complement in promoting primary humoral responses dates back over a quarter of a century. However, it is only in the course of the past decade or so that the detailed mechanisms underlying complement's influence have been characterized in depth. It is now clear that com- plement serves as a regulator of several B cell functions, including specific antibody production, antigen uptake, processing and presentation, and shaping of the B cell repertoire. Of key impor- tance, in this respect, is the role played by the B cell-signaling triad consisting of the B cell receptor for antigen (BCR), a complex composed of the iC3b/C3d fragment-binding complement type 2 re- ceptor (CR2, CD21) and its signaling element CD19 and the IgG-binding receptor FcRIIb (CD32). The positive or negative outcome of sig- naling through this triad is determined by the con- text in which antigen is seen, be it alone or in association with natural or induced antibodies and/or C3-complement fragments. The aim of this review is to describe the present status of our un- derstanding of complement's participation in ac- quired immunity and the regulation of autoimmune responses. J. Leukoc. Biol. 72: 249-261; 2002.