Cytokine that stimulates the proliferation of T-lymphocytes. Stimulation by IL-15 requires interaction of IL-15 with components of IL-2R, including IL-2R beta and probably IL-2R gamma but not IL-2R alpha (By similarity). In neutrophils, stimulates phagocytosis probably by signaling through the IL15 receptor, composed of the subunits IL15RA, IL2RB and IL2RG, which results in kinase SYK activation (By similarity). Belongs to the IL-15/IL-21 family.