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.