Natural killer cell: A type of immune cell that has granules (small particles) with enzymes that can kill tumor cells or cells infected with a virus. A natural killer cell is a type of white blood cell. NK cells, like killer T-cells, attack and kill cancer cells and cells infected by microorganisms. NK cells can react against and destroy another cell without prior sensitization to that cell. Natural killer (NK) cells are part of our first line of defense against cancer cells and virus-infected cells. NK cells are small lymphocytes that originate in the bone marrow.