The role of complement in the acquired immune response

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Abstract
Summary: Studies over the past three decades have clearly established a central role for complement in the promotion of a humoral immune response. The primary function of complement, in this regard, is to opsonize antigen or immune complexes for uptake by complement receptor type 2 (CR2, CD21) expressed on B cells, follicular dendritic cells (FDC) and some T cells. A variety of mechanisms appear to be involved in complement‐mediated promotion of the humoral response. These include: enhancement of antigen (Ag) uptake and processing by both Ag‐specific and non‐specific B cells for presentation to specific T cells; the activation of a CD21/CD19 complex‐mediated signalling pathway in B cells, which provides a stimulus synergistic to that induced by antigen interaction with the B‐cell receptor (BCR); and promotion of the interaction between B cells and FDC, where C3d‐bearing immune complexes participate in intercellular bridging. Finally, current studies suggest that CR2 may also play a role in the determination of B‐cell tolerance towards self‐antigens and thereby hold the key to the previously observed correlation between deficiencies of the early complement components and autoimmune disease.

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