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[Immunology](https://durgical.com/en/immunology/) · [Immune cells](https://durgical.com/en/immunology/#a-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

## Key points

- Markers: CD56 and CD16; no CD3 and no T-cell receptor.
- Missing self: inhibitory receptors read MHC class I; cells that lose MHC class I are killed.
- Kill with perforin and granzymes (and Fas ligand), inducing apoptosis of the target.
- CD16 binds IgG on coated cells: antibody-dependent cellular cytotoxicity (ADCC).
- Activated by IL-12, IL-15 and type I interferons; secrete interferon-γ that activates macrophages.

## 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

- The NK cell touches a body cell and reads both activating and inhibitory receptors.
- Normal MHC class I engages inhibitory receptors and stops the attack.
- A virus-infected or tumor cell shows stress ligands and less MHC class I.
- Activating signals win; the NK cell releases perforin and granzymes onto the target.
- 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.

## Memory aid

The NK cell checks ID cards: no MHC class I card, no entry, and the cell is removed on the spot.

## 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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## Related

- [CD8⁺ cytotoxic T cell](https://durgical.com/en/immunology/cd8-cytotoxic-t-cell/)
- [Innate immunity (overview, barriers)](https://durgical.com/en/immunology/innate-immunity/)
- [Interferon-γ](https://durgical.com/en/immunology/interferon/)
- [Type I interferons (α, β)](https://durgical.com/en/immunology/type-i-interferons/)
- [CD markers: the table to know](https://durgical.com/en/immunology/cd-markers-the-table-to-know/)

[← All of immunology](https://durgical.com/en/immunology/)

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](https://www.immunology.org/public-information/bitesized-immunology/cells/natural-killer-cells) (British Society for Immunology, 2024); [21.2 Barrier Defenses and the Innate Immune Response](https://openstax.org/books/anatomy-and-physiology-2e/pages/21-2-barrier-defenses-and-the-innate-immune-response) (OpenStax, Rice University (Anatomy and Physiology 2e), 2022); [18.4 Leukocytes and Platelets](https://openstax.org/books/anatomy-and-physiology-2e/pages/18-4-leukocytes-and-platelets) (OpenStax, Rice University (Anatomy and Physiology 2e), 2022).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/immunology/natural-killer-cell/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/immunology/natural-killer-cell/