Interleukin 15 (IL-15) is a cytokine that reveals multiple actions on immune system. Initially it was thought to resemble IL-2, but there is still growing evidence that IL-15 has multiple, unique properties, in some cases acting opposingly to IL-2. IL-15 influences mechanisms of both innate and acquired immunity. It induces differentiation and proliferation of NK cells, increases their cytotoxic properties and ability of IFN-γ secretion and also serves as chemoattractant for them. IL-15 acts also on macrophages/monocytes as well as on neutrophils.