Which arm of the acquired immune system is primarily mediated by T lymphocytes?

• A. Antibody-mediated
• B. Cell-mediated
• C. Humoral
• D. Complement-mediated

Answer: (B)

Cell-mediated immunity is the part of the acquired (adaptive) immune response driven mainly by T lymphocytes. T cells recognize antigen fragments presented on MHC molecules by other cells. Cytotoxic T cells (CD8+) directly kill infected or abnormal cells, while helper T cells (CD4+) coordinate the response by activating other immune cells, such as macrophages and cytotoxic T cells. This pathway is crucial for defending against intracellular pathogens, cancer, and in transplant rejection. In contrast, antibody-mediated (humoral) immunity is driven by B cells that produce antibodies targeting extracellular pathogens and toxins, and complement-mediated responses involve plasma proteins that enhance immunity but are not the primary mechanism controlled by T cells. Therefore, the arm primarily mediated by T lymphocytes is cell-mediated immunity.