Figure 3. Feedback loop of the alternative pathway.

C3b is formed by spontaneous C3 turnover in blood or generated by the lectin or classical pathways. C3b bound to a substrate binds the serine protease factor B (B). This low-affinity complex is converted by the serine protease factor D (D) to a C3-cleaving enzyme, C3bBb. This enzyme complex is stabilized by properdin (P). C3bBbP then cleaves C3 to C3a and C3b to complete the loop and lead to amplification.