What are natural killer cells?

• A. Lymphocytes that kill tumor and infected cells.
• B. Antigen-presenting cells.
• C. B cells that produce antibodies.
• D. Cytotoxic T cells.

Answer: (A)

Natural killer cells are a type of lymphocyte in the innate immune system that can recognize and kill cells that are stressed, infected, or transformed into tumor cells, without needing prior exposure to a specific pathogen. They act early in an immune response, using mechanisms such as perforin and granzyme to trigger target cell death, and they can also mediate antibody-dependent cellular cytotoxicity by binding to antibodies on target cells through their Fc receptors. Unlike cytotoxic T cells, which are part of the adaptive immune system and require a specific antigen presented by MHC molecules to activate, natural killer cells do not rely on this antigen-specific recognition. They are not antigen-presenting cells, which are specialized to display antigen fragments to T cells, nor are they B cells that produce antibodies. So, natural killer cells are best described as lymphocytes that kill tumor and infected cells.