The 2A3 antibody reacts with human CD25, a 55 kDa type I integral transmembrane glycoprotein also known as low-affinity interleukin 2 receptor alpha chain (IL-2Rα) or Tac antigen. CD25 is present on activated T and B cells, monocytes, macrophages, and regulatory T cells. CD25 has a role in T lymphocyte activation and can be used in the identification of T cell subsets in viral infections like HIV and HTLV. CD25 associates with IL-2 receptor β (CD122) and common γ (CD132) chains to form a high-affinity IL-2R complex. The 2A3 antibody binds to an epitope on p55 at or near the IL-2 binding site and prevents high-affinity IL-2 binding to T lymphocytes. Daclizumab, a therapeutic humanized monoclonal antibody, blocks the binding of 2A3 antibody.