The alternative complement pathway can be activated by which of the following?

• A. Can be activated by bacterial capsule polysaccharides.
• B. Uses C5b as a C3 convertase.
• C. Bypasses steps C3 through C5 of the classical pathway.
• D. Is inactivated by properdin.

Answer: (A)

Activation of the alternative pathway happens directly on microbial surfaces, independent of antibodies. It starts with a small, spontaneous C3 breakdown product binding to the surface, and on a surface rich in bacterial capsule polysaccharides this C3b coating is favored. This C3b then binds factor B, which is cleaved by factor D to form the C3 convertase C3bBb on the surface. The C3 convertase rapidly amplifies C3 activation and can progress to a C5 convertase, leading to formation of the membrane attack complex. This makes capsule polysaccharides a classic trigger for initiating the alternative pathway.

The other statements don’t fit this mechanism. The alternative pathway does not use C5b as a C3 convertase—C5b appears after C5 is cleaved to drive the terminal steps. It does not bypass C3 and C5 steps; it still involves C3 activation and then C5 to form the downstream convertases. Properdin does not inactivate the pathway; it stabilizes the C3bBb convertase, prolonging its activity.