Which of the following describes the role of properdin in the alternative pathway?

• A. Conversion of B to Bb
• B. Stabilization of C3/C5 convertase
• C. Inhibition of C3 convertase formation
• D. Binding and cleavage of Factor B

Answer: (B)

Properdin acts as a positive regulator of the alternative pathway by stabilizing the C3 convertase on pathogen surfaces. When C3b binds Factor B and Factor D cleaves B to Bb, the C3 convertase is formed as C3bBb, but it is relatively unstable and tends to dissociate. Properdin binds to this C3bBb complex, markedly increasing its half-life and keeping it active longer. This stabilization allows more C3 to be cleaved, boosting amplification of the cascade and facilitating the formation of the C5 convertase and, on suitable surfaces, the membrane attack complex. So, stabilization of the C3/C5 convertase best describes properdin’s role. It does not convert B to Bb, nor does it inhibit C3 convertase formation or bind and cleave Factor B.