Which statement correctly describes the alternative pathway in complement activation?

• A. It can be activated by bacterial capsule polysaccharides.
• B. It relies on C1 to start activation.
• C. It requires antibodies of the IgM class to function.
• D. It is entirely independent of C3.

Answer: (A)

The alternative pathway is activated directly on a pathogen’s surface without the need for antibodies. It begins with a spontaneous tickover of C3 to C3b, and on a microbial surface this C3b can bind factor B, which is cleaved by factor D to form the C3 convertase C3bBb. This convertase then cleaves more C3, creating a positive feedback loop that amplifies complement activation. Surfaces such as bacterial capsule polysaccharides provide the kind of environment that promotes C3b deposition and stabilize the convertase, enabling activation even in the absence of antibodies. Because C3 activation is central to this pathway, statements implying dependence on C1 or antibodies or implying independence from C3 don’t fit.