Question at position 10 Which of the following complement fixation pathways can be initiated by a soluble C3 convertase?All of the answers are correct.AlternativeNone of the answers are correct.LectinClassicalSingle choice
A
All of the answers are correct.
B
Alternative
C
None of the answers are correct.
D
Lectin
E
Classical

Question

Question at position 10 Which of the following complement fixation pathways can be initiated by a soluble C3 convertase?All of the answers are correct.AlternativeNone of the answers are correct.LectinClassicalSingle choice

A

All of the answers are correct.

B

Alternative

C

None of the answers are correct.

D

Lectin

E

Classical

BlackTom AI · Accepted Answer

The alternative pathway uniquely generates a fluid-phase (soluble) C3 convertase, C3(H2O)Bb, via spontaneous C3 “tick-over.”

1) Relevant knowledge points and concepts
- Three complement activation pathways: classical, lectin, and alternative.  
- Each pathway forms a C3 convertase but the composition differs: classical/lectin produce C4b2a (surface-associated), while the alternative pathway produces C3bBb (surface) and a fluid-phase form C3(H2O)Bb.  
- Alternative pathway initiation occurs by spontaneous hydrolysis of C3 (tick-over), recruitment of factor B, cleavage by factor D, and formation of the soluble convertase.

2) Step-by-step reasoning process
- Spontaneous hydrolysis: native C3 ⇢ C3(H2O).  
- C3(H2O) binds factor B: C3(H2O)B.  
- Factor D cleaves bound factor B into Ba and Bb, leaving C3(H2O)Bb — the soluble C3 convertase.  
- C3(H2O)Bb cleaves additional C3 into C3a and C3b. C3b can covalently attach to nearby surfaces, leading to formation of surface-bound C3bBb (amplification loop) stabilized by properdin.

3) Intermediate steps shown
- C3 → spontaneous hydrolysis → C3(H2O)  
- C3(H2O) + factor B → C3(H2O)B  
- Factor D cleaves B → Ba + Bb → C3(H2O)Bb (soluble C3 convertase)  
- C3(H2O)Bb → cleaves C3 → C3a + C3b → opsonization/amplification

4) Why the correct answer is correct
The alternative pathway is the only one that produces this fluid-phase convertase through tick-over; this soluble convertase initiates complement activation without antibody or lectin recognition.

5) Why other options are incorrect
- Classical and Lectin pathways form C3 convertase C4b2a, generated after C1 (classical) or MBL/MASP (lectin) activation, and these are primarily surface-associated rather than produced as a soluble C3(H2O)Bb.  
- “All of the answers” and “None of the answers” are therefore incorrect.

Conclusion: The correct answer is Alternative, which matches the provided answer.

Question at position 10 Which of the following complement fixation pathways can be initiated by a soluble C3 convertase?All of the answers are correct.AlternativeNone of the answers are correct.LectinClassicalSingle choice

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