How is the alternative complement pathway activated?

• A. Antibodies through Fc
• B. Several complement proteins (factors) interact in the plasma
• C. Binding of MBL to carbohydrates
• D. C1 binds to antibodies

Answer: (B)

Activation of the alternative complement pathway is driven by spontaneous interactions among several complement proteins in the plasma that form an active C3 convertase on microbial surfaces. C3 can hydrolyze spontaneously to C3(H2O), which allows Factor B to bind. Factor D then cleaves B to Bb, creating the C3 convertase C3bBb. Properdin stabilizes this complex, boosting its activity and leading to rapid cleavage of more C3 into C3a and C3b. This amplification loop proceeds without antibodies or lectin binding and targets microbes, promoting opsonization and inflammation and eventually contributing to MAC formation. The other options describe pathways that require antibodies (classical) or lectin binding (lectin pathway), not the antibody-independent activation of the alternative pathway.