Natural Killer Cell (NK Cell)

Natural killer cells are innate lymphoid cells, identified as CD56-positive and CD3-negative and making up roughly five to fifteen per cent of circulating lymphocytes. They kill by releasing perforin and granzymes, and they secrete interferon gamma. Unlike T cells they carry no rearranged antigen receptor; targeting rests on an arithmetic of signals, with inhibitory receptors reading MHC class I on the target while activating receptors read stress-induced ligands. A cell that sheds class I to escape CD8 T cells becomes a natural killer target.

A third function matters clinically: CD16 binds the Fc region of IgG, so these cells perform antibody-dependent cellular cytotoxicity and contribute to the effect of antibodies such as rituximab and trastuzumab. Function is measured by cytotoxicity against the K562 cell line, historically by chromium-51 release and now by flow cytometry. People with rare natural killer cell deficiencies suffer severe recurrent herpesvirus infections, the cleanest evidence of what these cells do.

That assay is where most claims go wrong. Cytotoxicity is reported as per cent lysis at a given effector-to-target ratio and moves for reasons unrelated to health. A single bout of intense exercise sharply raises circulating natural killer cell numbers by demargination, then drops them below baseline within hours; adrenaline alone reproduces the effect.

A study reporting increased natural killer activity after a supplement therefore has to account for sampling time, exercise, stress and diurnal variation, and show a change in function per cell rather than in cell count. Immune-peptide marketing routinely cites raised cytotoxicity in vitro as though it predicted fewer infections or slower tumour growth, an inference no controlled human outcome data supports.