Interleukin-2 receptor subunit alpha (IL2RA) is also known as IL-2R subunit alpha, IL-2-RA, IL2-RA, TAC antigen, p55, CD antigen CD25, is a type I transmembrane glycoprotein. The interaction of interleukin-2 (IL-2) with its receptor (IL-2R) critically regulates the T-cell immune response, and the α chain CD25/IL-2Rα is required for the formation of the high-affinity receptor. Among them, IL-2 plays a major role in the regulation of the magnitude and duration of T-cell activation in the immune responses. IL-2 promotes T-cell proliferation by interacting with its high-affinity receptor composed of three transmembrane polypeptides, the α, β, and γc chains. The α chain (CD25) is induced during T-cell activation, and its association with β and γc chains constitutes the high-affinity receptor. Most of the biologic effects of IL-2 are mediated through the high-affinity complex as evidenced by the almost identical phenotype of mice with targeted inactivation of the genes for IL-2 and IL-2Rα.