Feedback loop of the alternative complement pathway

After C3b is generated, it binds to factor B. Factor D can then cleave factor B to form the C3 convertase (C3bBb). The attachment of properdin (P) stabilizes the complex. The end result is the deposition of large amounts of C3b on the pathogen. C3b can be generated by the classical pathway, the lectin pathway, or de novo by the alternative pathway itself. A small amount of autoactivated C3 [called C3(H₂O)] is always present and may also bind factor B to engage the alternative pathway.