Medicine & USMLE

Natural Killer (NK) Cells

Other Cell Types
  1. Neutrophils - Overview
  2. Neutrophils - Granules
  3. Neutrophils - Oxidative Burst
  4. Basophils
  5. Eosinophils
  6. Mast Cells
  7. Monocytes / Macrophages
  8. Natural Killer (NK) Cells
  9. Dendritic Cells


Natural Killer (NK) cells are a type of innate immune cell of lymphoid origin. Being part of the innate immune system, NK cells recognize and kill unhealthy cells by looking for non-specific abnormalities. NK cells are stimulated by interleukin-2, interleukin-12, and interferons alpha and beta. NK cells function by attacking or killing cells lacking MHC I, which is down-expressed by virally-infected and cancer cells. In addition, NK cells can also attack cells that are bound by IgG antibodies, in what is called antibody-dependent cell-mediated cytotoxicity (ADCC). In order to kill a target cell, NK cells release perforin and granzyme. Perforins open holes in the target cell membrane, which granzyme enters to induce cell death.

Key Points

  • Natural Killer Cell (NK Cells)
    • Lymphocyte of the innate immune system
      • No antigen-specific activities; do not require exposure to an antigen for activation and do not possess antigen memory
    • Stimulated by
    • Produces Interferon gamma (IFN-gamma)
    • Activated by non-specific surface signals
      • Cells lacking MHC I
        • Seen in stress, cancer/malignancy, or viral infections
      • Cells with bound Immunoglobulin G (IgG)
        • Referred to as antibody-dependent cell-mediated cytotoxicity (ADCC)
        • CD16 expressed on NK Cells binds Fc region of bound IgG
    • Activation releases granules containing perforin and granzyme
      • Perforin forms pores in target cell membrane
      • Granzyme enters through these pores to induce cell death