IL-15 receptor alpha (IL-15Ralpha) is a cytokine receptor with the structural homology to IL-2Ralpha (CD25). Both alpha receptors contain a group of beta sandwich protein called sushi domains. Similar to IL-2Ralpha, IL-15Ralpha is able to form a heterotrimer with their beta chain (CD122) and common gamma chain (gammac, CD132). But unlike IL-2Ralpha subunit, IL-15Ralpha itself binds to IL-15 with equivalent high affinity compared with the heterotrimeric complex. IL-15Ralpha is expressed as soluble or transmembrane forms by variety cells, such as activated monocytes, dendritic cells, CD8+ memory T cells, NK or NKT cells. IL-15Ralpha by itself forms stable complexes with IL-15 on cell surface and presents IL-15 in trans to neighboring cells. IL-15, like IL-2, is a pleiotropic four-helix bundle cytokine and plays a key role in promoting T and B cell survival and proliferation, preferentially in memory CD8+ T cell proliferation, and NK/NKT cell development.