CD25, also known as the interleukin-2 receptor alpha chain （IL-2Rα）, is a membranous glycoprotein that forms the alpha chain of the IL-2 receptor complex. It is primarily expressed on activated T lymphocytes and plays a crucial role in regulating immune responses by mediating IL-2 signaling. When combined with IL-2Rβ and the common γ chain, CD25 forms a high-affinity receptor complex that enhances the binding and signaling of IL-2, promoting T-cell proliferation and activation. Additionally, CD25 is involved in the development and function of regulatory T cells, which help maintain immune tolerance and prevent autoimmune diseases. Abnormal expression of CD25 has been associated with various diseases, including autoimmune disorders and certain cancers, making it a potential target for therapeutic interventions.