Fig. 3

Feedback loop of the alternative pathway. Following the attachment of C3b to its target, a feedback loop can be engaged via interactions with the two proteases, factor B (FB) and factor D (FD), to form the AP C3 convertase. The binding of properdin (P) stabilizes the complex (i.e., its half-life is increased from 30–40 s to 3–4 min). Within a few minutes, more than one million C3bs can be generated and bound to a single bacterium