Fig. 9. Overview of complement activation pathways. The classical pathway is initiated by C1 binding to antigen-antibody complexes, and the alternative pathway is initiated by C3b binding to various activating surfaces, such as microbial cell walls. The C3b involved in alternative pathway initiation may be generated in several ways, including spontaneously, by the classical pathway, or by the alternative pathway itself. Both pathways converge and lead to the formation of the membrane attack complex. In this and subsequent figures, bars over the letter designations of complement components indicate enzymatically active forms and dashed lines indicate proteolytic activities of various components. (After Abbas AK, Lichtman AH, Pober JS. Cellular and Molecular Immunology, p 296. 2nd ed. Philadelphia: WB Saunders, 1994.)