||Interleukin 15 (IL-15) is a cytokine with structural similarity to IL-2 that is secreted by mononuclear phagocytes (and some other cells) following infection by virus(es). This cytokine induces cell proliferation of natural killer cells; cells of the innate immune system whose principal role is to kill virally infected cells. IL-15 was discovered to be a member of a family of cytokines, which also includes IL-2, IL-4, IL-7, IL-9, and IL-21. The specific receptor for IL-15, IL-15 receptor alpha, appears to regulate IL-15 secretion, stability, and activity. All members of this family use the common gamma chain as part of their signaling complex. IL-15 has been shown to play a role in many diseases, including obesity, influenza, and inflammatory autoimmune diseases.