Q.27
The activation of the complement components occurs via three distinct pathways.
Which of the following component(s) is specific to the Alternate Pathway?
Options:
(A) Factor B and D
(B) Mannose binding protein
(C) C1q r2 s2
(D) C2
The complement system activates via three pathways—Classical (antibody-dependent), Lectin (mannose-binding), and Alternate (spontaneous C3 hydrolysis)—converging at C3 cleavage for opsonization, inflammation, and membrane attack complex formation. Alternate Pathway uniquely relies on serum proteins Factor B and Factor D for its C3 convertase.
The correct answer is (A) Factor B and D, exclusive to Alternate Pathway activation on pathogen surfaces without antibodies.
Why (A) Factor B and D Are Alternate Pathway Specific
Alternate Pathway begins with spontaneous C3 → C3(H₂O) hydrolysis, exposing a Factor B binding site. Factor B binds C3b, gets cleaved by Factor D (serine protease) into Bb + Ba, forming C3bBb (Alternate C3 convertase). Properdin (Factor P) stabilizes this tick-over amplification loop unique to this pathway.
Explanation of All Options
Each represents different pathway initiators:
-
(A) Factor B and D
Correct. Form C3 convertase (C3bBb) in Alternate Pathway. No role in Classical/Lectin pathways. -
(B) Mannose binding protein (MBP)
Lectin Pathway specific. Binds microbial mannose → MASP activation → C4bC2b convertase. Antibody-independent but not Alternate. -
(C) C1q r2 s2 (C1qrs complex)
Classical Pathway initiator. C1q binds IgG/M → activates C1r → C1s → C4bC2a convertase. Antibody-dependent. -
(D) C2
Shared by Classical + Lectin pathways. Cleaved into C2a + C2b; combines with C4b for C4bC2a convertase. Absent from Alternate.
Quick Pathway Comparison Table
| Option | Component | Pathway Specificity | Convertase Formed |
|---|---|---|---|
| A | Factor B, D | Alternate ONLY | C3bBb |
| B | Mannose binding protein | Lectin ONLY | C4bC2b |
| C | C1qr₂s₂ | Classical ONLY | C4bC2a |
| D | C2 | Classical + Lectin | C4bC2a |
Biotech relevance: Alternate Pathway dysregulation causes autoimmune diseases (C3 glomerulopathy). Factor B/D inhibitors in clinical trials for complementopathies. Memory trick: “B&D = Bacteria Defense” (Alternate Pathway’s pathogen surveillance).
