IL-2Rα(Activated T cells IL-2Rα Chain 55), also kown as membrane interleukin-2 receptor (mIL-2R) or CD25, is a protein encoded by IL2RA gene in Human. This protein is 55 kDa which is the high affinity composition of IL-2R. Few IL-2α chains will express in inactive T cells, while the expression will remarkably increase followed by IL-2 accumulation, when T cell surface receptor (TCR) is active. IL-2αchains will become soluble interleukin-2 receptor (sIL-2R) when they are detached from cell membrane into serum, and the molecular weight of sIL-2R is 45 kDa. The concentration of sIL-2R in serum can be used for a mark to measure T cells activation and IL-2Rα chains expression.