The CD25 (HI25a) antibody recognizes a 55 kDa type-I transmembrane glycoprotein known as the low-affinity receptor for IL-2 (IL-2Rα). CD25 associates with CD122 (IL-2Rβ, P75) and the common γ Chain-CD132 (IL-2Rγ, P64) to form the high-affinity IL-2R complex. CD25 antigen is expressed on activated T and B lymphocytes, activated macrophages, and adult T-cell leukemia (ATL) cells, but is absent from resting T and B cells. CD25 antigen density is altered by treatment with PHA, ConA and CD3 antibody.