Which cell type is primarily responsible for cell-mediated cytotoxicity?

• A. Cytotoxic T cells
• B. Helper T cells
• C. Regulatory T cells
• D. Memory T cells

Answer: (A)

Cytotoxic T lymphocytes are the main effectors of cell-mediated cytotoxicity. These CD8+ T cells recognize antigen fragments presented on MHC class I molecules on infected or abnormal cells. When activated, they release perforin to form pores in the target cell membrane and granzymes to trigger apoptosis, directly killing the threat. Helper T cells aid in activating cytotoxic T cells and coordinating the response but do not directly kill target cells themselves. Regulatory T cells modulate the response to prevent autoimmunity, not mediate killing. Memory T cells provide quicker responses upon re-exposure but are not the immediate cytotoxic effectors. So the primary cell type responsible is the cytotoxic T lymphocyte.