GLOSSARY

A natural killer cell is a lymphocyte that contributes to innate defence against some infected or abnormal cells. It integrates signals from activating and inhibitory receptors on its surface. The combined signal determines whether the cell kills a target, secretes mediators or remains inactive.

Recognition and action

• Inhibitory signals: Normal expression of selected MHC class I molecules can restrain killing through inhibitory receptors.
• Activation: Stress ligands or the binding of an NK-cell receptor to antibody on a target can favour activation.
• Effector mechanisms: Perforin and granzymes can help trigger death of a target. NK cells can also produce interferon gamma.

How it differs from a cytotoxic T cell

Conventional cytotoxic T cells use a specific T-cell receptor to recognize a peptide on MHC. NK cells rely on a different set of receptors and can respond without that specific antigen recognition. An NK cell can bind the Fc region of antibody coating a target through receptors such as CD16. This connects an antibody response to a cellular effector mechanism. It differs from recognition based only on signals originating on the target cell.

A practical comparison

Blood NK-cell counts can rise during intense exercise and fall during recovery. A lower circulating count at a later sampling point can reflect redistribution and does not by itself demonstrate weaker antiviral defence.