CD25, also known as the interleukin-2 receptor alpha chain (IL2RA), is a type I transmembrane glycoprotein that plays a crucial role in the immune system. It is primarily expressed on the surface of activated T cells and regulatory T cells (Tregs). CD25 is a key component of the high-affinity IL-2 receptor complex, which also includes the IL-2 receptor beta and gamma chains. By itself, CD25 can bind IL-2 with low affinity, but when combined with the beta and gamma chains, it forms a high-affinity receptor that initiates intracellular signaling pathways such as MAPK and JAK/STAT. This receptor complex is essential for the proliferation and activation of T cells, as well as the regulation of immune tolerance by controlling the activity of Tregs. Mutations in the IL2RA gene can lead to deficiencies in IL-2 receptor function, highlighting its importance in maintaining immune homeostasis.