Reacts with CD25, a 55 kDa glycoprotein known as the low-affinity interleukin-2 receptor (IL-2R). CD25 is expressed on activated lymphocytes (T and B) and monocytes, and associates with the p75 high-affinity receptor chain to form the IL-2R complex. The CD25 molecule reveals three epitope regions: A, B, and C. M-A251 antibody recognizes epitope region B. CD25 expression on lymphocytes is upregulated by anti-CD3 or PHA stimulation. Soluble IL-2R is produced following inflammatory responses. This antibody is routinely tested by flow cytometric analysis. Synonyms: IL-2 Receptor alpha Chain