OX40 is a T cell co-stimulatory molecule of the TNF receptor superfamily that coordinates with other co-stimulators (CD28, CD40, CD30, CD27 and 4-1BB) to manage the activation of the immune response. Tumor necrosis factor ligand superfamily member 4 (TNFSF4) is also known as glycoprotein Gp34, OX40 ligand (OX40L), TAX transcriptionally-activated glycoprotein 1 and CD252, which belongs to the tumor necrosis factor family. TNFSF4 is the ligand for CD134 and is expressed on cells such as DC2s (a subtype of dendritic cells) enabling amplification of Th2 cell differentiation. OX40 is constitutively expressed on Tregs and enhances the sensitivity of Tregs to IL-2, thus promoting Treg proliferation. OX40 has also shown to decrease the cells’ immunosuppressive activity on effector T cells. OX40-OX40 Ligand signaling is involved in allergic airway inflammation, graft-versus-host disease and autoimmune disease.