Which statement accurately describes a function of Factor H?

Which statement accurately describes a function of Factor H?

• A. Acts with DAF to promote breakdown of C3b
• B. Prevents binding of Factor B to C3b
• C. Binds to C3b to promote C3 convertase activity
• D. Binds to C3b to promote Factor I mediated cleavage

Answer: (D)

Factor H regulates the alternative pathway by binding to C3b on cell surfaces and acting as a cofactor for Factor I. This pairing enables Factor I to cleave C3b into inactive fragments (iC3b), which cannot support formation of the C3 convertase. By inactivating C3b, Factor H halts further amplification of the complement cascade on host tissues. While Factor H can also influence convertase formation by reducing Factor B binding to C3b, the essential description is its role as a cofactor for Factor I–mediated cleavage of C3b.

Factor H regulates the alternative pathway by binding to C3b on cell surfaces and acting as a cofactor for Factor I. This pairing enables Factor I to cleave C3b into inactive fragments (iC3b), which cannot support formation of the C3 convertase. By inactivating C3b, Factor H halts further amplification of the complement cascade on host tissues. While Factor H can also influence convertase formation by reducing Factor B binding to C3b, the essential description is its role as a cofactor for Factor I–mediated cleavage of C3b.