Interleukin-15 is a member of the four α helix bundle family of cytokines. It plays a multifaceted role in the development and control of the immune system. The IL-15R consists of three polypeptides, the type-specific IL-15Rα, the IL-2/IL-15Rβ, and the common γ-chain (γC), which is shared by multiple cytokine receptors. IL-15 could enhance the effect of chemotherapeutic agents and ACT on transplanted tumors, but had little or no effect when administered alone. The result of research shows that the biological activity of IL-15 could be increased ∼50-fold by administering preformed complexes of IL-15 and its soluble receptor, IL-15Rα.