Question

The cytolytic activity of the membrane attack complex is modulated by?

• A. Factor I
• B. Factor B
• C. Factor S
• D. Factor H

Hint:

Vitronectin, the S protein, blocks membrane insertion of the C5b-7 complex.

Answer 1

The Correct Option is C

Step 1: The membrane attack complex (MAC, C5b-9) forms pores in target cell membranes. Several regulators control complement to protect host tissues.

Step 2: Factor S, also called vitronectin, binds the fluid-phase C5b-7 complex and prevents its insertion into the membrane, thereby modulating and limiting MAC cytolytic activity. This is the regulator that acts directly on the terminal lytic pathway.

Step 3: Factor I is a protease that cleaves C3b and C4b. Factor H is a cofactor that helps Factor I and accelerates decay of the alternative pathway C3 convertase. Factor B is a component of the alternative pathway C3 convertase. None of these acts on the MAC itself.

Step 4: The correct answer is option 3, Factor S.