Antibody production in humoral immunity is primarily carried out by which cells?

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

Answer: (B)

Humoral immunity relies on B lymphocytes that can become antibody-secreting plasma cells. When a B cell encounters its specific antigen and receives help from helper T cells, it becomes activated and differentiates into plasma cells that produce and release large amounts of antibodies tailored to that antigen. These antibodies circulate in the blood and lymph, neutralize pathogens, tag them for phagocytosis (opsonization), and help activate the complement system, all of which work together to clear extracellular invaders. Some activated B cells become memory B cells for quicker responses in the future. The other cells listed—cytotoxic T cells, macrophages, and natural killer cells—play roles in different aspects of immunity (cell-mediated responses, antigen presentation and phagocytosis, and killing infected or abnormal cells, respectively) but do not primarily produce antibodies.