Question

Opsonin is

• A. C3a
• B. C3b
• C. C5a
• D. C6

Hint:

Think of the complement fragment that binds the bacterial surface for phagocyte uptake, not the anaphylatoxins.

Answer 1

The Correct Option is B

Step 1: Opsonization is the process of coating a foreign particle so that it is recognized and prepared for phagocytosis. The molecules that do this coating are called opsonins.
Step 2: The main opsonin derived from the complement system is C3b. Other recognized opsonins include antibodies (IgG, IgA) and complement fragments C4b and iC3b. C3 is the most abundant complement protein and is cleaved into C3a and C3b.
Step 3: C3b is the most critical fragment in both the classical and alternative pathways. It binds directly to bacterial surfaces and is recognized by phagocyte receptors, driving opsonization. Hence the answer is C3b.
Step 4: Why the distractors are wrong: C3a is an anaphylatoxin that activates mast cells and basophils to release histamine, not an opsonin. C5a is a potent chemotactic and anaphylatoxic fragment, not an opsonin. C6 is part of the membrane attack complex (C5b-C9) that causes cell lysis, not opsonization.