Q.64 Choose the correct statements from those given below:

A. C5 convertase is formed from C5a and C5b convertase
B. C5 convertase is formed from C3b and C3 convertase
C. C3a and C3b are formed by C5 convertase
D. The membrane attack complex consist of C5b, C6, C7, C8 and C9
E. C5 convertase is formed from C5ab

Choose the correct answer from the options given below:

1. B and E only
2. C and B only
3. A and C only
4. B and D only

   The correct answer is B and D only.

   C5 convertase forms by adding C3b to C3 convertase, and the membrane attack complex (MAC) indeed includes C5b through C9, making statements B and D accurate in the complement cascade.

   Statement Evaluation

   A. C5 convertase is formed from C5a and C5b convertase
   False. C5a is an anaphylatoxin fragment from C5 cleavage, not a component of convertases; C5b initiates MAC, and no “C5b convertase” exists.

   B. C5 convertase is formed from C3b and C3 convertase
   True. C3 convertase (classical: C4b2a; alternative: C3bBb) gains C3b to become C5 convertase (C4b2a3b or C3bBb3b), switching specificity for C5 cleavage.

   C. C3a and C3b are formed by C5 convertase
   False. C3 convertase cleaves C3 into C3a/C3b; C5 convertase targets C5 only.

   D. The membrane attack complex consist of C5b, C6, C7, C8 and C9
   True. C5b sequentially binds C6-C9 to form the pore-forming MAC (C5b-9).

   E. C5 convertase is formed from C5ab
   False. “C5ab” is not a recognized complement component or fragment.

   Option Breakdown

   Option	Statements	Correct?	Reason
   B and E only	B (true), E (false)	No	E invalid. ​
   C and B only	C (false), B (true)	No	C incorrect. ​
   A and C only	A (false), C (false)	No	Both wrong. ​
   B and D only	B (true), D (true)	Yes	Matches cascade facts. ​

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   C5 convertase formation occurs when C3b joins C3 convertase, enabling C5 cleavage to launch the membrane attack complex (C5b-C9) in the complement system’s terminal pathway—crucial for exams like NEET Life Sciences.

   Complement C5 Convertase Mechanics

   C3 convertase (classical: C4bC2b; alternative: C3bBb) cleaves C3 into C3a/C3b. Additional C3b deposition creates high-affinity C5 convertase (C4bC2bC3b or C3bBbC3b), cleaving C5 into C5a (anaphylatoxin) and C5b. This shift ensures amplification on pathogen surfaces.

   Membrane Attack Complex Assembly

   C5b nucleates MAC by binding C6, then C7 (anchors to membrane), C8 (initiates polymerization), and multiple C9 units forming a transmembrane pore (~100 Å) that lyses cells. This lytic pathway defends against bacteria and infected cells.

   Common Exam Misconceptions

   Avoid confusing convertase substrates: C3 convertase → C3; C5 convertase → C5. No involvement of C5a/C5b in convertase formation or “C5ab.” B and D correctly highlight C5 convertase from C3b/C3 convertase and MAC components.

   Study Tips for Immunology

   • Classical/lectin: C4b2a → C4b2a3b (C5 convertase).

   • Alternative: C3bBb → C3bBb3b.

   • Practice assertion questions on pathway enzymes for GATE/CSIR.​