What does Factor P bind to on microbial surfaces to prevent inhibition?

Factor P, also known as properdin, plays a crucial role in the immune response by stabilizing the alternative pathway C3 convertase, which is composed of C3b and Bb. By binding to microbial surfaces, Factor P enhances the activity of the C3 convertase, facilitating the opsonization of pathogens and promoting their clearance by phagocytic cells. This binding is essential because it prevents the degradation of the C3 convertase by regulatory proteins that could inhibit the complement activation needed to target pathogens effectively.

In contrast, Factor H is a regulatory protein that inhibits the alternative pathway by promoting the decay of C3 convertase and facilitating the cleavage of C3b. Human cell surface proteins and membrane co-factor protein are also involved in regulating complement activation but do not serve the function of stabilizing the C3 convertase like Factor P does. Thus, among the options provided, the binding of Factor P to C3 convertase on microbial surfaces is what allows the immune system to effectively combat infections.