OX40 Ligand (CD252, TNFSF4) is a member of the TNF superfamily that is present on the surface of antigen presenting cells, activated B lymphocytes, and other cells such as endothelial and mast cells. OX40L interacts with OX40 (CD134), a late activation-induced antigen which is expressed primarily on activated T cells. This costimulatory interaction leads to increased proliferation and cytokine production responses of activated T cells, and at the same time enhances proliferation and immunoglobulin secretion by activated B cells. OX40/OX40L ligation promotes survival and expansion of CD4 and CD8 T cells and the OX40/OX40L pathway is an emerging target for immunotherapy studies.