Which cells are natural killer cells, a type of lymphocyte that destroys abnormal cells via non-specific mechanisms?

• A. B cells
• B. Natural killer cells
• C. Dendritic cells
• D. T cells

Answer: (B)

Natural killer cells are cytotoxic lymphocytes that belong to the innate immune system and kill abnormal or stressed cells without needing prior exposure to a specific antigen. They recognize target cells by signs like reduced MHC class I or stress-induced ligands and release perforin and granzymes to induce apoptosis. They can also attack antibody-coated targets through their Fc receptor, enabling antibody-dependent cellular cytotoxicity. This non-specific, rapid cytotoxic action sets them apart from B cells, which produce antibodies (humoral immunity), T cells, which are adaptive and require antigen-specific activation, and dendritic cells, which are antigen-presenting cells that help activate other immune cells rather than directly killing targets. Therefore, the described cells are natural killer cells.