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The binding of complement component C3d or the final breakdown product of C3, C3dg, to CR2 (CD21) enhances antibody responses in several ways (Figure 4.7Ai) and discussed in Chapter 9. First, C3dg binds to antigen that is also bound to B‐cell surface Ig (see Figure 4.7A). C3dg can bind simultaneously to CR2, which is part of the B‐cell co‐receptor (see Chapter 9). Signaling through both the surface Ig and the co‐receptor augments activation of the B cell. Thus, C3dg binding to antigen and the B‐cell surface lowers the threshold for B‐cell activation by as much as 1000‐fold compared to binding in the absence of C3dg.

Figure 4.6. Major functions of complement: (A) production of opsonins; (B) production of anaphylatoxins; and (C) pathogen lysis.

Second, follicular dendritic cells in the germinal center bind antigen–antibody complexes and present antigen to proliferating B cells. This interaction is critical for the eventual development of memory cells. Follicular dendritic cells express the complement receptors CR2, which binds C3dg, and CR1, which binds iC3b. Thus follicular dendritic cells can present antigen–antibody complexes bound to one of these complement components to germinal center B cells (see Figure 4.7Aii). In this way, complement components also play a role in the induction of B‐cell memory.

In addition, B‐cell processing of T‐dependent antigens is more rapid when the antigen is bound to C3dg than when it is not; presumably the binding of C3dg to CR2 on the B‐cell surface enhances uptake and processing of the antigen. This may be another way in which complement enhances B‐cell responses to T‐cell‐dependent antigens.