Q.16 What is the composition of membrane attack complex in complement systems?
- C1 – C2 – C9 – C3 – C6 – C7
- C5b – C3b – C2 – C5a – C4b
- C3b – C2 – C4b – C1 – C3
- C5b – C6 – C7 – C8 – C9
The membrane attack complex (MAC) in the complement system is composed of C5b, C6, C7, C8, C9, forming a pore that lyses target cells.
Question Breakdown
This question assesses immunology knowledge of the terminal complement pathway, crucial for microbiology and molecular biology students studying innate immunity and pathogen clearance for exams like GATE Life Sciences.
Option Explanations
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C1 – C2 – C9 – C3 – C6 – C7: Incorrect; C1, C2, C3 initiate classical pathway but don’t form MAC pores; order is jumbled and excludes C8.
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C5b – C3b – C2 – C5a – C4b: Wrong; C3b/C4b/C2 are early opsonins/convertases, C5a is anaphylatoxin (not structural); MAC starts post-C5 cleavage.
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C3b – C2 – C4b – C1 – C3: Invalid; lists activation components only, omitting terminal MAC proteins (C5b-9); no pore-forming sequence.
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C5b – C6 – C7 – C8 – C9: Correct; sequential assembly: C5b binds C6→C7 (membrane insertion)→C8 (stabilizes)→multiple C9 (polymerizes ~18 into β-barrel pore, ~100 Å diameter).
Introduction to MAC Composition
The composition of membrane attack complex in complement systems is C5b-6-7-8-9, the terminal cytolytic pore from all pathways (classical/lectin/alternative). It punches holes in pathogen membranes for innate immunity.
MAC Assembly Sequence
C5 convertase cleaves C5→C5b (initiation). C5b+C6→C5b6 (soluble)→+C7 (C5b67 inserts hydrophobically)→+C8 (C5b8 penetrates bilayer)→+C9n (oligomerizes into transmembrane channel, causing colloid-osmotic lysis).
Options Comparison
Option Components Role in Complement MAC Accuracy C1-C2-C9-C3-C6-C7 Early + late mix Initiation only Wrong sequence C5b-C3b-C2-C5a-C4b Opsonins/anaphylatoxin Pre-terminal Excludes C6-9 C3b-C2-C4b-C1-C3 Classical activators C3 convertase No terminal pore C5b-C6-C7-C8-C9 Terminal proteins Pore formation Correct MAC C9 polymerization creates the lethal defect.
Immunology Applications
Targets for therapeutics (eculizumab blocks C5); key in bacterial lysis, transplant rejection; aligns with graduate-level innate immunity and complement deficiencies.
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