NATURAL KILLER CELLS

(NK cells). A type of lymphocyte that does not carry the markers to be B cells or T cells. Like cytotoxic T cells, they attack and kill tumor cells and protect against a wide variety of infectious microbes. They are "natural" killers because they do not need additional stimulation or need to recognize a specific antigen in order to attack and kill. Persons with immunodeficiences such as those caused by HIV infection have a decrease in "natural" killer cell activity. See also Antigen; B Lymphocytes; Cytotoxic; Lymphocyte; Null Cell; T Cells.