CD4 and HIV infectious disease: In addition to the reduced activating receptor expression on NK cells and appearance of poorly functional CD56−CD16+ NK cells in peripheral blood, HIV infection reduces the expression on CD4+ T and other target cells of ligands of activating NK cell receptors which are important in triggering NK cell cytotoxicity and cytokine production (e.g., PVR, NKp46-L, and NKp30-L) (73–76).