Factor B binds to surface-associated C3b C3b binds to cell surface C3(H2O):Factor Bb cleaves C3 to C3b and C3a Factor D cleaves C3(H2O)-bound Factor B Factor B binds to C3(H2O) Spontaneous hydrolysis of C3 thioester Formation of alternate C5 convertase Factor D cleaves C3b-bound Factor B Properdin stabilizes C3b:Bb bound to cell surfaces PathwayStep6820 PathwayStep6818 PathwayStep6819 PathwayStep6821 PathwayStep6822 PathwayStep6823 PathwayStep6824 PathwayStep6825 PathwayStep6826 Alternative complement activation Authored: de Bono, B, 2004-08-04 09:17:50 Edited: Jupe, S, 2010-11-17 GENE ONTOLOGYGO:0006957 ISBN0781735149 Pubmed162484 Reactome Database ID Release 43173736 Reactome, http://www.reactome.org ReactomeREACT_8001 Reviewed: D'Eustachio, P, 2006-07-03 21:35:13 The proteins participating in alternative pathway activation are C3 (and C3b), the factors B, D, and properdin. In the first place, alternative pathway activation is a positive feedback mechanism to increase C3b. When C3b binds covalently to sugars on a cell surface, it can become protected. Then Factor B binds to C3b. In the presence of Factor D, bound Factor B is cleaved to Ba and Bb. Bb contains the active site for a C3 convertase. Properdin then binds to C3bBb to stabilize the C3bBb convertase on cell surface leading to cleavage of C3. Finally, a C3bBb3b complex forms and this is a C5 convertase.