Natural killer cells are part of which arm of the immune system?

• A. Adaptive immunity
• B. Innate immunity
• C. Passive immunity
• D. Humoral immunity

Answer: (B)

Natural killer cells are part of the innate arm of the immune system. They provide rapid, nonspecific defense against virally infected and transformed cells without needing prior exposure to a specific antigen. NK cells can detect stressed or abnormal cells, such as those with reduced MHC class I expression, and kill them by releasing perforin and granzymes. They also secrete cytokines like interferon-gamma to help activate other immune cells. While they can interact with the adaptive system (for example, via antibody-dependent cellular cytotoxicity), their primary role is immediate, nonspecific defense rather than antigen-specific memory. In contrast, adaptive immunity relies on antigen-specific T and B lymphocytes with clonal expansion and memory, passive immunity involves transfer of antibodies or cells from another individual, and humoral immunity is the antibody-mediated response by B cells.