Humoral immunity refers to which of the following?

• A. Production of antibody by plasma cells
• B. Production of cytokines by T cells
• C. Elimination of virally infected cells by cytotoxic cells
• D. Downregulation of the immune response

Answer: (A)

Humoral immunity is the antibody-mediated arm of the adaptive immune response. It centers on B lymphocytes that recognize soluble antigens and, after activation, differentiate into plasma cells that secrete antibodies. These antibodies circulate in blood and extracellular fluids, neutralize pathogens and toxins, promote opsonization, and activate the classical complement pathway. This contrasts with cell-mediated immunity, which relies on T cells to kill infected cells or help other immune cells; cytokine production by T cells is part of that system, and cytotoxic T cells destroy virally infected cells. Downregulation of the immune response is a regulatory process, not the primary function of humoral immunity. Therefore, the production of antibody by plasma cells best reflects humoral immunity.