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Immunology · Immune cells

Natural killer cell !

  • Cell

NK cells are innate lymphocytes that kill virus-infected and tumor cells without prior sensitization, especially cells that have lost MHC class I.

Updated 2026-11-01

What it is

Natural killer (NK) cells are large granular lymphocytes of the innate immune system. They do not have a T-cell or B-cell receptor; instead they weigh activating signals against inhibitory receptors that recognize MHC class I on healthy cells. Infected or cancerous cells that lower MHC class I lose that protection and are killed, which covers the gap left when viruses hide from CD8 T cells.

Key facts

  • Origin: lymphoid progenitor in the bone marrow; they are a minority of blood lymphocytes.
  • Act within hours to days, before T-cell responses are ready.
  • They belong to the innate lymphoid cell family.

Step by step

  1. The NK cell touches a body cell and reads both activating and inhibitory receptors.
  2. Normal MHC class I engages inhibitory receptors and stops the attack.
  3. A virus-infected or tumor cell shows stress ligands and less MHC class I.
  4. Activating signals win; the NK cell releases perforin and granzymes onto the target.
  5. The target dies by apoptosis, and the NK cell can move on to another target.

Markers

  • CD56
  • CD16 (FcγRIII)
  • No CD3
  • Perforin and granzymes

Clinical notes

  • Rare NK-cell deficiencies cause severe, recurrent herpesvirus infections (varicella, cytomegalovirus, Epstein-Barr).
  • Chédiak-Higashi syndrome impairs NK and neutrophil granules.
  • ADCC by NK cells contributes to the effect of some therapeutic monoclonal antibodies against tumors.

Test yourself

Think of the answer, then tap the question to see it.

Which surface markers identify NK cells?

CD56 and CD16, without CD3 or a T-cell receptor.

What is the missing-self hypothesis?

NK cells kill cells that have lost MHC class I, because MHC class I normally engages their inhibitory receptors.

How do NK cells perform antibody-dependent cellular cytotoxicity?

Their CD16 (Fc receptor) binds IgG coating a target cell, triggering release of perforin and granzymes.

Which cytokine do NK cells secrete to activate macrophages?

Interferon-γ.

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Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given.

Sources: Natural Killer Cells (British Society for Immunology, 2024); 21.2 Barrier Defenses and the Innate Immune Response (OpenStax, Rice University (Anatomy and Physiology 2e), 2022); 18.4 Leukocytes and Platelets (OpenStax, Rice University (Anatomy and Physiology 2e), 2022).

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Representative model, for educational purposes only.