The Complement System cHAPTER 13 VISUALIZING CONCEPTS Clq binds antigen-bound Clr ac The C3b component of C5 convertase binds C5, permitting nd Clr: bot CIs C4b2a to cleave c5 Ciqr252 Antibody clr. C5 convertase Cls cleaves C4 and C2. Cleaving C4 exposes the binding site for c2. C4 binds the surface near Cl and c2 binds C4 forming C3 convertase C5b binds C6, initiating the formation of the membrane- attack C2 C3 convertase C567 C5b678 C3 convertase hydrolyzes many C3 molecules. Some combine ith C3 convertase to form C5 convertase C5b678 Poly-C9 C3a Membrane attack comple C5 convertase FIGURE 13-5 Schematic diagram of intermediates in the classi- attack complex(MAC, bottom right) forms a large pore in th cal pathway of complement activation. The completed membrane- membraneThe Complement System CHAPTER 13 303 VISUALIZING CONCEPTS FIGURE 13-5 Schematic diagram of intermediates in the classi￾cal pathway of complement activation. The completed membrane￾attack complex (MAC, bottom right) forms a large pore in the membrane. Poly-C9 3 5 C5b binds C6, initiating the formation of the membrane-attack complex C1s cleaves C4 and C2. Cleaving C4 exposes the binding site for C2. C4 binds the surface near C1 and C2 binds C4, forming C3 convertase C3 convertase hydrolyzes many C3 molecules. Some combine with C3 convertase to form C5 convertase 4 The C3b component of C5 convertase binds C5, permitting C4b2a to cleave C5 1 C1q binds antigen-bound antibody. C1r activates auto￾catalytically and activates the second C1r; both activate C1s + C3 convertase C5 convertase C5 convertase + C5 C5b C5a C1q FC Antibody C1r2s2 C4b2a C4b2a C4b2a3b C1qr2s2 C4a C2 C2b C4 C3 C3b C3a 2 C9 C5b678 C8 C5b C567 C5b678 C6 C7 Membrane attack complex