C3b and C4b Binding Facilitates Opsonization

C3b is the major opsonin of the complement system, although C4b and iC3b also have opsonizing activity. The amplification that occurs with C3 activation results in a coating of C3b on immune complexes and particulate antigens. Phagocytic cells, as well as some other cells, express complement receptors (CR1, CR3, and CR4) that bind C3b, C4b, or iC3b (see Table 13-4). Antigen coated with C3b binds to cells bearing CR1. If the cell is a phagocyte (e.g., a neutrophil, monocyte, or macrophage), phagocytosis will be enhanced (Figure 13-12). Activation of phagocytic cells by various agents, including C5a anaphylatoxin, has been shown to increase the number of CR1s from 5000 on resting phagocytes to 50,000 on activated cells, greatly facilitating their phagocytosis of C3b-coated antigen. Recent studies indicate that complement fragment C3b acts as an adjuvant when coupled with protein antigens. C3b targets the antigen directly to the phagocyte, enhancing the initiation of antigen processing and accelerating specific antibody production.

C3b Macrophage Receptor

FIGURE 13-12 (a) Schematic representation of the roles of C3b and antibody in opsonization. (b) Electron micrograph of Epstein-Barr virus coated with antibody and C3b and bound to the Fc and

C3b receptor (CR1) on a B lymphocyte. [Part (b) from N. R. Cooper and G. R. Nemerow, 1986, in Immunobiology of the Complement System, Academic Press.]

FIGURE 13-12 (a) Schematic representation of the roles of C3b and antibody in opsonization. (b) Electron micrograph of Epstein-Barr virus coated with antibody and C3b and bound to the Fc and

FIGURE 13-12

C3b receptor (CR1) on a B lymphocyte. [Part (b) from N. R. Cooper and G. R. Nemerow, 1986, in Immunobiology of the Complement System, Academic Press.]

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  • Alda Genovesi
    What facilitates opsonization?
    7 years ago

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