Factor H regulates the alternative pathway by competing with which molecule for the same binding site on C3b?

Factor H regulates the alternative pathway by competing with which molecule for the same binding site on C3b?

• A. Factor D
• B. Factor I
• C. C3b
• D. Factor B

Answer: (D)

Factor H works as a regulator of the alternative pathway by binding to C3b and preventing the assembly of the C3 convertase. The crucial step for amplifying the pathway is binding of Factor B to C3b to form the C3bBb complex, with Factor D then activating B to Bb. When Factor H occupies the same C3b binding site, Factor B cannot attach, so the C3bBb convertase cannot form and the amplification loop is dampened. Factor H also helps inactivate C3b via Factor I, but the key competitive interaction is with Factor B. That’s why the molecule competing for C3b at this site is Factor B.

Factor H works as a regulator of the alternative pathway by binding to C3b and preventing the assembly of the C3 convertase. The crucial step for amplifying the pathway is binding of Factor B to C3b to form the C3bBb complex, with Factor D then activating B to Bb. When Factor H occupies the same C3b binding site, Factor B cannot attach, so the C3bBb convertase cannot form and the amplification loop is dampened. Factor H also helps inactivate C3b via Factor I, but the key competitive interaction is with Factor B. That’s why the molecule competing for C3b at this site is Factor B.