Natural killer cell
Innate immune cells that kill stressed cells lacking self markers.
Natural killer cells, also known as NK cells, are a type of cytotoxic lymphocyte critical to the innate immune system. They are a kind of large granular lymphocyte (LGL), belong to the rapidly expanding family of known innate lymphoid cells (ILC), and represent 5–20% of all circulating lymphocytes in humans. NK cells provide rapid responses to virus-infected cells, stressed cells, tumor cells, and other intracellular pathogens based on signals from several activating and inhibitory receptors, and they recognize and kill stressed cells that are lacking MHC class I molecules.
- field
- Immunology
- known_for
- Natural cytotoxicity against tumor and virus-infected cells without prior sensitization
- role
- Cytotoxic lymphocyte of the innate immune system
- surface_markers
- CD56+, CD3−, CD16+, CD57+
- subtypes
- CD56bright and CD56dim
Lore & Background
The discovery that a unique type of lymphocyte was responsible for 'natural' or spontaneous cytotoxicity was made in the early 1970s by doctoral student Rolf Kiessling and postdoctoral fellow Hugh Pross, in the mouse, and by Hugh Pross and doctoral student Mikael Jondal in the human. The mouse and human work was carried out under the supervision of professors Eva Klein and Hans Wigzell, respectively, of the Karolinska Institute, Stockholm. Using discontinuous density centrifugation, and later monoclonal antibodies, natural killing ability was mapped to the subset of large, granular lymphocytes known today as NK cells.
Reader's Guide
Natural killer cells are significant as a critical component of the innate immune system, providing rapid responses to virus-infected cells, stressed cells, tumor cells, and other intracellular pathogens. Their ability to recognize and kill cells lacking MHC class I molecules fills a gap in immune surveillance that T cells cannot address. Outside of innate immunity, both activating and inhibitory NK cell receptors play important functional roles in self tolerance and the sustaining of NK cell activity. NK cells also play a role in the adaptive immune response, as experiments have demonstrated their ability to formulate antigen-specific immunological memory. The role of NK cells in both the innate and adaptive immune responses is becoming increasingly important in research using NK cell activity as a potential cancer therapy and HIV therapy. Their discovery resolved earlier inconsistencies in observations of natural reactivity against tumors, and the K562 51chromium-release assay became a standard method for detecting human NK functional activity, allowing comparison across laboratories worldwide.
Did You Know?
- NK cells represent 5–20% of all circulating lymphocytes in humans.
- They were named 'natural killers' because they do not require activation to kill cells missing 'self' markers of MHC class I.
- NK cells can be identified by the presence of CD56 and the absence of CD3 (CD56+, CD3−).
- The K562 51chromium-release assay is the most commonly used assay to detect human NK functional activity.
Frequently Asked Questions
Who is Natural killer cell?
NK cells are cytotoxic lymphocytes of the innate immune system, making up roughly 5–20% of all circulating lymphocytes in a human. They are classified as large granular lymphocytes and sit within the broader innate lymphoid cell family.
What are Natural killer cell's powers/role?
NK cells can spot and destroy virus-infected, stressed, or tumor cells almost instantly, without ever needing a prior sensitization round. They make the kill-or-spare call by cross-referencing signals from several activating and inhibitory receptors on their surface.
How does Natural killer cell's story end?
Once a target is flagged, the NK cell dumps its cytotoxic granules into the stressed cell, triggering apoptosis and eliminating the threat. The NK cell itself generally walks away unharmed and keeps patrolling for the next suspicious cell.
Why is Natural killer cell important?
Because they act without prior exposure, NK cells fill the critical gap before adaptive T cells and antibodies can organize a response. They are especially vital against cells that have stripped away MHC class I molecules to dodge T-cell surveillance.
What are Natural killer cell's surface markers and subtypes?
The canonical surface profile is CD56+, CD16+, CD57+, and CD3− (no T-cell receptor). Two main subsets exist—CD56bright and CD56dim—differing largely in how much cytokine production versus direct cytotoxicity each favors.
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