Question:easy
Which complement component acts as an opsonin?
Which complement component acts as an opsonin?
Show Hint
The larger cleavage fragment of C3 coats bacteria for phagocytosis.
Updated On: Jun 24, 2026
- C3a
- C3b
- C5a
- C6
Show Solution
The Correct Option is B
Solution and Explanation
Step 1: Define opsonisation as flagging a microbe with a molecular coat so phagocytes can grab it more easily. We need the complement fragment that does the coating.
Step 2: Complement C3 is split into two pieces, $C3a$ and $C3b$. The larger $C3b$ binds covalently to the bacterial surface and is the principal complement opsonin, which makes $C3b$ the answer.
Step 3: Supporting opsonins include iC3b and C4b, with antibody IgG doing the same job via Fc receptors.
Step 4: Separate the functions of the wrong options: $C3a$ and $C5a$ are anaphylatoxins driving inflammation and chemotaxis, while $C6$ joins the membrane attack complex for lysis, so none of them serves as an opsonin.
\[\boxed{\text{C3b}}\]
Step 2: Complement C3 is split into two pieces, $C3a$ and $C3b$. The larger $C3b$ binds covalently to the bacterial surface and is the principal complement opsonin, which makes $C3b$ the answer.
Step 3: Supporting opsonins include iC3b and C4b, with antibody IgG doing the same job via Fc receptors.
Step 4: Separate the functions of the wrong options: $C3a$ and $C5a$ are anaphylatoxins driving inflammation and chemotaxis, while $C6$ joins the membrane attack complex for lysis, so none of them serves as an opsonin.
\[\boxed{\text{C3b}}\]
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