Humoral immunity primarily mediated by?

• A. B lymphocytes and antibodies produced by plasma cells.
• B. Cytotoxic T lymphocytes.
• C. Macrophages.
• D. Natural killer cells.

Answer: (A)

Humoral immunity is antibody-mediated. It centers on B lymphocytes that recognize antigens and, after activation, differentiate into plasma cells that secrete antibodies into the blood and other body fluids. These antibodies bind specific pathogens or toxins, neutralize them, block their attachment to cells, promote their removal by phagocytes through opsonization, and can activate the complement system. This is distinct from cellular immunity, which is driven by T lymphocytes—cytotoxic T cells specifically kill infected cells—while macrophages and natural killer cells play other roles in immunity. Therefore, the mediators of humoral immunity are B lymphocytes and the antibodies produced by plasma cells.