Question at position 20 Which of the following provides serine protease activity in the classical complement activation pathway?Both C1r and C1s act in the classical pathway.C1rMASP-2None of the answers acts in the classical pathway.C1s单项选择题
A
Both C1r and C1s act in the classical pathway.
B
C1r
C
MASP-2
D
None of the answers acts in the classical pathway.
E
C1s

Question

Question at position 20 Which of the following provides serine protease activity in the classical complement activation pathway?Both C1r and C1s act in the classical pathway.C1rMASP-2None of the answers acts in the classical pathway.C1s单项选择题

A

Both C1r and C1s act in the classical pathway.

B

C1r

C

MASP-2

D

None of the answers acts in the classical pathway.

E

C1s

BlackTom AI · Accepted Answer

C1 complex of the classical pathway contains C1q, C1r and C1s; both C1r and C1s are serine proteases and provide the proteolytic activity for the classical complement cascade.

1. Relevant knowledge points and concepts
- The classical complement C1 complex = C1q (recognition) + C1r and C1s (proteases).  
- C1r and C1s are serine protease zymogens that become activated upon antibody (IgG/IgM)–C1q binding.  
- MASP-2 is a serine protease in the lectin pathway (associated with MBL), not the classical pathway.

2. Step-by-step reasoning process
- C1q binds Fc regions of antigen-bound IgM or clustered IgG.  
- C1q binding induces conformational changes that allow C1r to autoactivate (zymogen → active serine protease).  
- Active C1r then cleaves and activates C1s.  
- Activated C1s cleaves C4 and C2 to form the classical C3 convertase (C4b2a).

3. Intermediate mechanistic steps (cleavages)
- C1r (inactive) → C1r (active) by autoactivation after C1q engagement.  
- Active C1r cleaves C1s (zymogen → active C1s).  
- C1s cleaves C4 → C4a + C4b; C4b binds surface.  
- C1s cleaves C2 → C2a + C2b; C2a associates with C4b → C4b2a (C3 convertase).

4. Why the correct answer is correct
- Both C1r and C1s have serine protease activity in the classical pathway: C1r is the initial activator (autoactivates and activates C1s) and C1s executes the downstream proteolytic cleavages of C4 and C2. Together they provide the pathway’s serine protease activity.

5. Why other options are incorrect
- C1r (alone): although a serine protease, C1r’s primary role is to activate C1s; both act rather than only C1r.  
- C1s (alone): C1s performs C4/C2 cleavage but requires activation by C1r; the pathway employs both.  
- MASP-2: belongs to the lectin pathway (MBL-associated serine protease), not the classical pathway.  
- None of the answers: incorrect because classical pathway proteolysis is mediated by C1r/C1s.

6. Conclusion
Both C1r and C1s act in the classical pathway (correct answer).

Question at position 20 Which of the following provides serine protease activity in the classical complement activation pathway?Both C1r and C1s act in the classical pathway.C1rMASP-2None of the answers acts in the classical pathway.C1s单项选择题

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