Natural killer cells

What do they do? NK cells are equipped with cytotoxic granules that contain the arsenal of cytotoxic molecules used in killing virally infected and cancerous cells. They recognize their targets, in part, by means of activating signals that are only just beginning to be identified, but also by detecting cells that are deficient in cell-surface class I molecules of the major histocompatibility complex (class I MHC molecules). Tumour cells and cells infected with viruses have often lost their cell-surface MHC molecules, which makes them susceptible to attack. Healthy cells displaying self-MHC molecules are protected against attack by NK cells (see Figure).