Definition of Natural killer cell

Natural killer cell: A cell that can react against and destroy another cell without prior sensitization to it. 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 and develop fully in the absence of the thymus. NK cells look for a "banner" flown by normal cells. If the NK cell recognizes the "banner," it spares that cell. If the "banner" is absent, the NK cell attaches to the target cell, releases a burst of chemicals that penetrate the target cell's wall, and the target cell breaks up (lyzes).