Human Interleukin-15 (IL-15) is a widely expressed 14 kDa cytokine that is plays an important role in many immunological diseases. It shares approximately 97% and 73% sequence identity with simian and murine IL-15, respectively. Both human and simian IL-15 are active on murine cells. IL-15 is secreted by mononuclear phagocytes (and some other cells), especially macrophages following infection by virus. IL-15 and IL-2 have similar biologic properties in vitro, consistent with their shared receptor (R) signaling components (IL-2/15Rβγc). A role for IL-15 during autoimmune processes such as rheumatoid arthritis and malignancies such as adult T-cell leukemia suggest that dysregulation of IL-15 may result in deleterious effects for the host.