CD25 is a protein found on the surface of certain immune cells, particularly T cells. It is also known as the interleukin-2 receptor alpha chain and plays a crucial role in the immune response. CD25 helps regulate the activity of T cells, which are essential for fighting infections and maintaining immune system balance. In addition to its role in normal immune function, CD25 is often associated with regulatory T cells (Tregs), which help prevent autoimmune diseases by suppressing excessive immune responses. The presence of CD25 can be used as a marker to identify these regulatory cells in research and clinical settings.