Which component is primarily mediator of cellular immunity?

• A. B cells
• B. NK cells
• C. Antibodies
• D. T cells

Answer: (D)

Cellular immunity is driven by T lymphocytes, which directly target infected or abnormal cells and coordinate other immune responses. Cytotoxic T cells (CD8+) detect intracellular antigens presented on MHC class I and destroy the threat by releasing perforin and granzymes to induce cell death. Helper T cells (CD4+) recognize antigens on MHC class II and secrete cytokines that activate macrophages, support the activation of B cells, and boost cytotoxic T cell responses. In contrast, B cells are central to humoral immunity, producing antibodies that neutralize pathogens in body fluids, while natural killer cells belong to the innate immune system and kill some stressed or infected cells without antigen-specific activation. Thus, T cells are the primary mediators of cellular immunity.