art 1 Immune effector mechanisms VISUALIZING CONCEPTS C3 hydrolyzes spontaneously, C3b agent attaches to foreign surface Factor B binds C3a, exposes site ac Factor B on by Factor D. Cleavage generates C3bBb, which has c3 convertase Binding of properdin stabilizes convertase C3 convertase Convertase generates C3b: some binds to C3 convertase activating C convertase. C5b binds to antigenic C3bBb3B surface C3 convertase Membrane FIGURE 13.7 Schematic diagram of intermediates in the for- erdin. Conversion of bound C5b to the membrane-attack complex mation of bound C5b by the alternative pathway of complement occurs by the same sequence of reactions as in the classical path- activation. The C3b Bb complex is stabilized by binding of prop- way(see Figure 13-5) complex or other noncellular activating surface, then the hy- complexes mediate tissue damage will be considered in drophobic binding sites cannot anchor the complex and it is Chapter 20 released. Released C5b67 complexes can insert into the mem Binding of C8 to membrane-bound C5b67 induces a con- brane of nearby cells and mediate"innocent-bystander"lysis. formational change in C8, so that it too undergoes a hy Regulator proteins normally prevent this from occurring, but drophilic-amphiphilic structural transition, exposing a in certain diseases cell and tissue damage may result from in- hydrophobic region, which interacts with the plasma mem nocent-bystander lysis a hemolytic disorder resulting from brane. The C5b678 complex creates a small pore, 10 A in di deficiency in a regulatory protein is explained in the Clinical ameter formation of this pore can lead to lysis of red blood Focus section and an autoimmune process in which immune cells but not of nucleated cells. The final step in formation ofcomplex or other noncellular activating surface, then the hy￾drophobic binding sites cannot anchor the complex and it is released. Released C5b67 complexes can insert into the mem￾brane of nearby cells and mediate “innocent-bystander” lysis. Regulator proteins normally prevent this from occurring, but in certain diseases cell and tissue damage may result from in￾nocent-bystander lysis. A hemolytic disorder resulting from deficiency in a regulatory protein is explained in the Clinical Focus section and an autoimmune process in which immune 306 PART III Immune Effector Mechanisms complexes mediate tissue damage will be considered in Chapter 20. Binding of C8 to membrane-bound C5b67 induces a con￾formational change in C8, so that it too undergoes a hy￾drophilic-amphiphilic structural transition, exposing a hydrophobic region, which interacts with the plasma mem￾brane. The C5b678 complex creates a small pore, 10 Å in di￾ameter; formation of this pore can lead to lysis of red blood cells but not of nucleated cells. The final step in formation of VISUALIZING CONCEPTS 3 2 4 1 Factor B binds C3a, exposes site acted on by Factor D. Cleavage generates C3bBb, which has C3 convertase activity Binding of properdin stabilizes convertase Convertase generates C3b; some binds to C3 convertase activating C5' convertase. C5b binds to antigenic surface C3 hydrolyzes spontaneously, C3b fragment attaches to foreign surface + Properdin + C3 + C3 C3b C3a + C5 C5b Membrane attack complex C5a Factor B Factor D C3 convertase C3bBb C3bBb3B C3 convertase FIGURE 13-7 Schematic diagram of intermediates in the for￾mation of bound C5b by the alternative pathway of complement activation. The C3bBb complex is stabilized by binding of prop￾erdin. Conversion of bound C5b to the membrane-attack complex occurs by the same sequence of reactions as in the classical path￾way (see Figure 13-5)