Key Protein in Complement Activation

Q66.Key protein involved in complement activation is (1) C5 (2) C8 (3) C9 (4) C3

Q66.Key protein involved in complement activation is

(1) C5
(2) C8
(3) C9
(4) C3

Key Protein in Complement Activation: C3 Explained

C3 stands out as the central protein driving complement activation across all pathways. This multiple-choice question tests core immunology knowledge relevant to exams like NEET or USMLE.

Correct Answer

The correct option is (4) C3.​

C3 acts as the convergence point for classical, lectin, and alternative pathways, where C3 convertases cleave it into C3a and C3b. C3b amplification fuels downstream activation, including opsonization and further convertase formation.

Option Breakdown

Option Role in Complement Why Not Key for Activation?
(1) C5 Cleaved by C5 convertases into C5a (anaphylatoxin, chemotactic) and C5b (initiates MAC). Activation occurs late after C3; C5 convertases depend on prior C3b deposition.
(2) C8 Binds C5b-7 complex and recruits C9 to form MAC pores. ​ Terminal pathway component; no enzymatic activation role. ​
(3) C9 Polymerizes into MAC transmembrane channels for lysis. Final effector; activation completes post-C3/C5 steps. ​
(4) C3 Central hub; cleaved to amplify cascade, form C5 convertases (e.g., C4b2b3b, C3bBbC3b). Drives all pathways to terminal phase. ​

Why C3 Dominates

C3’s cleavage marks the “point of no return,” generating over 80% of downstream effectors via positive feedback. Dysregulation links to diseases like atypical hemolytic uremic syndrome, making it a therapeutic target.

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