Natural killer cell. Coloured scanning electron micrograph (SEM) of a natural killer cell (NK) (blue) attacking a liver cancer cell. NK cells of a healthy individual were cultured with hepatoma (HepG2) cells. Natural killer cells, also known as large granular lymphocytes (LGL), are a type of cytotoxic lymphocyte critical to the innate immune system that belong to the rapidly expanding family of known innate lymphoid cells (ILC) and represent 5??0% of all circulating lymphocytes in humans. NK cells provide rapid responses to virus-infected cell and other intracellular pathogens acting at around 3 days after infection, and also respond to tumor formation. Magnification x 7000 at 10cm wide. Specimen courtesy of the Maini Lab. UCL.