CD25, also known as the alpha chain of the interleukin-2 receptor (IL-2Rα), is a protein marker predominantly found on the surface of activated T cells, B cells, and regulatory T cells (Tregs). It plays a crucial role in the immune response by binding to interleukin-2, which is vital for T cell proliferation and immune cell signaling. CD25 is also used as a marker to identify and characterize Tregs, which are essential in maintaining immune system homeostasis and preventing autoimmune reactions. High expression of CD25 is often associated with immune activation, and its levels can be indicative of various immune-related conditions, including autoimmune diseases and cancer. In the context of cancer, CD25 can be a potential target for immunotherapy, as it is highly expressed on Tregs that can suppress anti-tumor immune responses. Targeting CD25 may help in depleting Tregs and enhancing the effectiveness of immune treatments.