CD25 is the alpha chain of the IL-2 receptor. It is a type I transmembrane protein present on activated T cells, activated B cells, some thymocytes, myeloid precursors, and oligodendrocytes that associates with CD122 to form a heterodimer that can act as a high-affinity receptor for IL-2. Though CD25 has been shown to influence effector function of lymphocytes, it is thought to play a greater role in immune tolerance in mice. Constitutive expression of CD25 on Treg cells is thought to be crucial to their survival and maintenance of immune homeostasis. Expression of CD25 and its role in immunology may be species dependent, since CD25 appears to play a larger role in T-cell effector responses in humans compared to mice, and may be somewhat dispensable for the maintenance of Treg cells as seen in patients treated with CD25-blocking antibodies.