The complement system is classified as which type of immunity?

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

Answer: (C)

The complement system is a set of soluble proteins circulating in body fluids that mediate defense without requiring immune cells. This makes it part of the humoral branch of immunity—the noncellular, fluid components that act in extracellular environments. When activated, these proteins opsonize pathogens, promote inflammation, and can directly lyse microbes through the membrane attack complex. Although the classical pathway involves antibodies, there are also antibody-independent routes (lectin and alternative), showing it links with antibodies but remains a fluid-phase system. It’s not a cellular process, and it isn’t the adaptive arm driven by antigen-specific receptors, which is why it’s categorized with humoral immunity.