Natural killer cells are large granular lymphocytes of the innate immune system. They identify and kill abnormal cells (virus-infected, tumor-precursor) without needing antibody activation.
Mechanism — Recognize 'missing self' (cells that downregulate MHC class I to hide from T cells) and 'stress markers.' Release perforin and granzymes to induce target apoptosis. Activity tracked via lab assays.
Use case — Support via exercise (acute and chronic), sufficient sleep, vitamin D, zinc, beta-glucans, mushroom polysaccharides. Chronic stress, alcohol, and sleep deprivation drop activity sharply.
Caveats — NK activity testing is specialized. Symptomatic chronic viral reactivation (EBV, CMV) often coincides with low NK function — pair clinical context with labs.