Cytokine that stimulates the proliferation of T-lymphocytes. Stimulation by IL15 requires interaction of IL15 with components of the IL2 receptor, including IL2RB and probably IL2RG but not IL2RA (By similarity). In neutrophils, stimulates phagocytosis probably by signaling through the IL15 receptor, which results in kinase SYK activation (By similarity). Expressed in many tissues including heart, spleen, lung, liver, muscle and kidney (at mRNA level). Expressed in many tissues including heart, spleen, lung, liver, muscle and kidney (at protein level). Belongs to the IL-15/IL-21 family.