Usually pertaining to Natural Killer Cells. Natural Killer Cells are an important first line of defense against newly arising malignant cells and cells infected with viruses, bacteria, and protozoa. They form a distinct group of lymphocytes with no immunological memory and are independent of the adaptive immune system. Natural Killer Cells constitute 5 to 16 percent of the total lymphocyte population. Their specific function is to kill infected and cancerous cells.