Interleukin 15 (IL-15) has been described as a cytokine with biologic functions similar to those of IL-2. IL-2 and IL-15 utilize a receptor composed of three subunits: IL-2 receptor β-chain, the y-chain (βc) shared by IL-15, IL-2, IL-4, IL-7, and IL-9 receptors, and a unique a-chain. The α-chain of the IL-15 receptor (IL-15Rα) and IL-2Rα share a fragmentary sequence homology and structural similarities, and both proteins contain a conserved “sushi domain” binding motif. IL-15 is a potent T-cell activator that induces the effector function of cytotoxic T cells and lymphokine-activated killer cells. IL-15, like IL-2, stimulates the proliferation of activated CD4, CD8, B cells, and natural killer (NK) cells (1 ).