The Formation of the C3 Convertase Is a Key Step

question 26

Multiple Choice

The formation of the C3 convertase is a key step in complement activation that occurs in all three complement pathways. This enzyme cleaves C3 in blood plasma, leading to a conformational change in the C3b fragment that exposes its reactive thioester group. The activated C3b is potentially harmful to the host, if it becomes covalently attached to a host cell, rather than to the surface of a pathogen. This deleterious outcome is largely avoided by:

• A) The inability of active C3b to diffuse away in the blood plasma.
• B) The inability of active C3b to covalently attach to the membranes of eukaryotic cells.
• C) The rapid hydrolysis of active C3b in solution, rendering it inactive.
• D) The tight binding of active C3b to the C3 convertase.
• E) The ability of active C3b to recruit phagocytic cells.

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