What is the role of the alternative complement pathway in host defense?

• A. It requires antigen-antibody complexes to activate
• B. It is activated on microbial surfaces independently of antibodies to opsonize and promote inflammation
• C. It produces interferons
• D. It primarily clears immune complexes

Answer: (B)

The role of the alternative complement pathway is to provide immediate, antibody-independent defense against microbes by activating directly on their surfaces to tag them for attack and summon inflammation. C3 undergoes spontaneous hydrolysis (tickover) and, on a microbial surface, forms C3 convertase with factor B and D, stabilized by properdin. This leads to deposition of C3b on the pathogen (opsonization) and release of inflammatory mediators like C3a and C5a, which recruit and activate immune cells. The pathway can also advance to form the membrane attack complex to lyse susceptible microbes. It works without prior antibodies, distinguishing it from pathways that rely on antigen-antibody complexes, and while it can contribute to immune complex clearance, its primary function is rapid, antibody-independent opsonization and inflammation on pathogens.