Natural Killer (NK) Cells are lymphocytes in the same family as T and B cells, coming from a common progenitor.
As cells of the innate immune system, NK cells are classified as group I Innate Lymphocytes (ILCs) and respond quickly to a wide variety of pathological challenges.
NK cells are best known for killing virally infected cells, and detecting and controlling early signs of cancer. As well as protecting against disease, specialized NK cells are also found in the placenta and may play an important role in pregnancy.
(Source: British Society for Immunology)