Activation of the Alternative Complement Pathway
and Formation of C3 Convertase

Activation of the alternative complement pathway begins when C3b (or C3i) binds to the cell wall and other surface components of microbes. Alternative pathway protein Factor B then combines with the cell-bound C3b to form C3bB. Factor D then splits the bound Factor B into Bb and Ba, forming C3bBb. A serum protein called properdin then binds to the Bb to form C3bBbP that functions as a C3 convertase capable of enzymatically splitting hundreds of molecules of C3 into C3a and C3b. Some C3b binds to C3bBb to form C3bBb3b. C3bBb3b functions as a C5 convertase that can enzymatically split hundreds of molecules of C5 into C5a and C5b.

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