CD25 is a 55 kD glycoprotein also known as IL-2 receptor alpha chain. It is broadly expressed on activated T and B cells, a subset of thymic and splenic dendritic cells, and intestinal epithelial cells in the rat. IL-2 is a critical cytokine involved in lymphocyte proliferation and clonal expansion. IL-2 signaling requires the high affinity IL-2 receptor composed of IL-2 receptor chains alpha, beta, and gamma. The OX-39 antibody weakly inhibits IL-2 binding to the IL-2 receptor and has been shown to block some IL-2 mediated responses in vitro.