Question:hard
The cytolytic activity of the membrane attack complex is modulated by?
The cytolytic activity of the membrane attack complex is modulated by?
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Vitronectin, the S protein, blocks membrane insertion of the C5b-7 complex.
Updated On: Jun 24, 2026
- Factor I
- Factor B
- Factor S
- Factor H
Show Solution
The Correct Option is C
Solution and Explanation
Step 1: Focus on which named factor regulates the terminal complement step that builds the lytic pore rather than the earlier convertase steps.
Step 2: Factor S, the protein vitronectin, attaches to nascent C5b-7 in the fluid phase so the complex cannot anchor to a membrane, blunting formation of C5b-9 and protecting bystander cells. This is the direct modulator of MAC-mediated cytolysis.
Step 3: The other choices act upstream: Factor I degrades C3b and C4b, Factor H is its cofactor and a decay accelerator of the alternative pathway, and Factor B feeds the alternative pathway convertase. They do not touch the assembled attack complex.
Step 4: Hence the modulator of MAC cytolytic activity is Factor S.
\[\boxed{\text{Factor S (vitronectin)}}\]
Step 2: Factor S, the protein vitronectin, attaches to nascent C5b-7 in the fluid phase so the complex cannot anchor to a membrane, blunting formation of C5b-9 and protecting bystander cells. This is the direct modulator of MAC-mediated cytolysis.
Step 3: The other choices act upstream: Factor I degrades C3b and C4b, Factor H is its cofactor and a decay accelerator of the alternative pathway, and Factor B feeds the alternative pathway convertase. They do not touch the assembled attack complex.
Step 4: Hence the modulator of MAC cytolytic activity is Factor S.
\[\boxed{\text{Factor S (vitronectin)}}\]
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