Recognizes a protein of 55kDa, identified as CD25 (Workshop IV; Code A27). CD25 is expressed on activated T- and B-cells and activated monocytes/macrophages. With respect to lymphomas, CD25 is present on malignant cells of Hodgkin’s disease, HTLV-1+ adult T-cell leukemia, cutaneous T-cell lymphoma, and hair cell leukemia. Increased levels of soluble CD25 are observed in the leukemias/lymphomas and inflammatory/ autoimmune diseases. CD25 molecule alone appears to function as a low affinity receptor and associates with CD122 (IL-2R ?? ? chain, p75) and CD132 (common ?? ? chain) to form the high affinity IL-2 receptor complex. CD25 antibodies detect three epitope regions, A, B and C. This MAb recognizes the epitope B, which is located at residue 3-104 of CD25 and can effectively block IL-2 binding to CD25.