In the alternative pathway of complement activation, spontaneous hydrolysis of C3 allows continuous low-level C3b generation. Which plasma protein binds and stabilizes the alternative pathway C3 convertase, amplifying the cascade on microbial surfaces?

• A Factor I
• B C4-binding protein
• C Factor H
• D Properdin (factor P) ✓

Explanation

Properdin is the only known positive regulator of complement; it stabilizes C3bBb, extending its half-life several-fold and permitting amplification on microbial surfaces where host regulators such as factor H are absent. Factor I with factor H degrades C3b, so options C and A describe inhibitory control proteins, while C4-binding protein regulates the classical pathway convertase C4b2a and plays no role in the alternative loop.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.