CD25, also known as the interleukin-2 receptor alpha chain (IL-2Rα), is a protein that serves as a receptor for interleukin-2 (IL-2), a cytokine that plays a crucial role in immune system regulation. CD25 is expressed on the surface of various immune cells, including T cells, B cells, natural killer (NK) cells, dendritic cells, and monocytes. The main function of CD25 is to bind to IL-2, which then triggers a signaling cascade that leads to the activation, proliferation, and differentiation of immune cells. CD25 expression is tightly regulated and is often used as a marker for regulatory T cells (Tregs), a subset of T cells that play a key role in suppressing immune responses and maintaining immune homeostasis. Tregs are characterized by high expression levels of CD25 and the transcription factor FoxP3. Abnormalities in CD25 expression or function have been linked to various immunological disorders. For example, deficiencies in CD25 expression or function can lead to immunodeficiency, while excessive CD25 expression has been associated with autoimmune diseases and cancer.