Question

Which of the following is the opsonin?

• A. C5b
• B. C1q
• C. C3a
• D. C3b

Hint:

C3b is the opsonin that enhances phagocytosis by marking pathogens for immune cell recognition.

Answer 1

The Correct Option is D

Step 1: Understanding Opsonins 
Opsonins are molecules that enhance phagocytosis by marking pathogens for recognition by immune cells like macrophages and neutrophils. 
Step 2: Evaluating the Options 
- C5b: Part of the complement system, involved in the formation of the membrane attack complex, not an opsonin. 
- C1q: Part of the complement system, helps initiate the classical pathway but not directly involved in opsonization. 
- C3a: Involved in inflammation but not an opsonin. 
- C3b: Correct, C3b is an opsonin that binds to pathogens, promoting their uptake by phagocytes. 
Step 3: Conclusion 
C3b acts as the primary opsonin in the immune response.