NK cells are important in protecting against viral infections, and they may regulate the immune response. They are activated by hematopoietic blasts and pose a barrier to bone marrow transplantation. They are also abundant in the pregnant uterine decidua, although their role there is unknown. NK cells are normally inhibited from responding to host cells by inhibitory receptors that recognize self class I MHC antigens. There is evidence that NK cells may be important in the regulation of autoimmunity, but there is even stronger evidence that NKT cells regulate autoimmunity. The mechanisms by which these cells are activated and by which they regulate other cells are now being understood at the molecular level.