Which T-cell subset is CD8+ and cytotoxic?

• A. Natural killer cells
• B. B cells
• C. Cytotoxic T cells
• D. Helper T cells

Answer: (C)

CD8+ cytotoxic T cells are the immune cells that directly kill infected or abnormal cells. They carry the CD8 co-receptor and become cytotoxic cytotoxic T lymphocytes when activated. They recognize peptide antigens presented by MHC class I molecules on almost all nucleated cells, which allows them to detect cells that are infected or transformed. Upon activation, they release perforin and granzymes to induce apoptosis in the target cell, and can also engage Fas-FasL pathways to trigger cell death. This is a hallmark of adaptive, cell-mediated immunity against intracellular pathogens and cancers.

Natural killer cells can be cytotoxic, but they are part of the innate immune system and do not require antigen-specific recognition through T-cell receptors or the CD8 marker. B cells are primarily antibody producers and do not function as cytotoxic T cells. Helper T cells provide coordination by helping activate other immune cells and carry the CD4 marker rather than CD8.