1. Survey cells
NK cells encounter potential targets and assess their surface ligands.
IMMUNE CELLS
Natural Killer
(NK) Cells
Survey. Recognize Stress. Eliminate Threats.
Natural killer cells are innate lymphocytes that help control infected and abnormal cells. They integrate activating and inhibitory signals, deliver cytotoxic molecules to selected targets and release cytokines that coordinate defense. Their responses are tightly regulated to limit injury to healthy tissues.
Back to all innate immune cells →What Are Natural Killer Cells?
Natural killer cells are lymphocytes with cytoplasmic granules containing perforin and granzymes. Unlike conventional T cells, they do not use a rearranged antigen-specific T-cell receptor. They can respond without prior antigen-specific sensitization, while still requiring appropriate activating signals.
NK cells assess the balance between stress-related ligands and inhibitory signals, including those associated with self MHC class I. Some also recognize antibody-coated targets through CD16. Their activity combines selective cell killing with cytokine production and interactions with other immune cells.
How NK Cells Recognize and Eliminate Targets
Target selection depends on integrated signals rather than one universal marker.
2. Integrate signals
Activating and inhibitory receptors together determine whether a response proceeds.
3. Deliver granules
A focused contact allows release of perforin and granzymes toward the selected target.
4. Induce apoptosis
Cytotoxic pathways can trigger controlled target-cell death; the NK cell can detach.
Key Functions of Natural Killer Cells
Specialized cellular functions contribute to coordinated immunity.
1. Support antiviral defense
NK cells can recognize and eliminate selected infected cells during early immune responses.
2. Survey abnormal cells
Stress ligands and reduced inhibitory signaling can make some tumor cells susceptible.
3. Coordinate immunity
Cytokines such as interferon-γ help shape responses by macrophages and other cells.
4. Use antibody-guided recognition
CD16 can bind antibody Fc regions on coated targets and support antibody-dependent killing.
Key Takeaways
- NK cells are innate lymphocytes with cytotoxic granules.
- Activating and inhibitory signals jointly guide target selection.
- Perforin and granzymes support controlled target-cell killing.
- Cytokines connect NK-cell activity with wider immune responses.
- CD16 enables recognition of some antibody-coated targets.
- NK-cell function varies with tissue, activation and cell state.
Natural Killer Cells and Aging
Aging can alter NK-cell populations, receptor patterns and functional responses.
- The balance of NK-cell subsets can change.
- Receptor expression and differentiation states may shift.
- Cytotoxic and cytokine responses are not uniformly affected.
- Persistent infections and tissue signals influence the repertoire.
- Functional changes differ between individuals and conditions.
Selected sources and references
- NK-cell morphology and recognition
- NK cells, antibodies and aging
- NK-cell phenotype and function in aging
- NK-cell repertoires across age
Artwork illustrates the main mechanisms schematically; molecular sizes, colors and distances are not literal.