The complement system is considered to be _____ immunity.

• A. Innate Immunity
• B. Cellular Immunity
• C. Adaptive Immunity
• D. Humoral Immunity

Answer: (D)

Humoral immunity refers to defenses that operate in body fluids with soluble factors rather than through direct cell-mediated actions. The complement system is a group of plasma proteins that circulate in blood and lymph and become activated in a cascade to defend against pathogens. Once activated, they can tag microbes for destruction (opsonization), form membrane attack complexes that lyse them, and recruit inflammatory cells to the site of infection. These are all fluid-phase, extracellular actions that supplement antibodies and can be triggered by them, but the effectors themselves are circulating proteins in the humoral compartment. That combination—soluble proteins in the body’s fluids working to neutralize and clear pathogens—fits the humoral aspect of immunity.