Human IL-2 receptor alpha (IL-2 R alpha), also known as Tac antigen and as CD25, was initially identified as a 55 kDa membrane glycoprotein that is capable of binding IL?2.eceptor for interleukin-2. The receptor is involved in the regulation of immune tolerance by controlling regulatory T cells (TREGs) activity. TREGs suppress the activation and expansion of autoreactive T-cells.