CD25 is a 55 kD type I transmembrane glycoprotein also known as low affinity IL-2 receptor alpha chain or Tac. It is expressed on progenitor lymphocytes, activated T and B cells, and activated monocytes/macrophages. CD25 is also expressed on a subset of non-stimulated CD4+ T cells termed T regulatory cells. Soluble CD25/IL-2Ralpha is produced as a consequence of lymphocyte stimulation and is found in biological fluids following inflammatory responses. CD25 associates with IL-2 receptor beta (CD122) and common gamma (CD132) chains to form a high affinity IL-2R complex.