OX40 (CD134; TNFRSF4) is a T cell costimulatory molecule of the TNF receptor superfamily that coordinates with other costimulators (CD28, CD40, CD30, CD27 and 4-1BB) to manage the activation of the immune response. OX40 is upregulated on antigen activated CD4+ and CD8+ T cells with costimulation by CD40-CD40 Ligand and CD28-B7. OX40 interactions with OX40 Ligand on antigen presenting cells enhances T cell survival, proliferation, and cytokine production. It also inhibits the conversion of effector T cells into regulatory T cells (Tregs) and can promote the maintenance of and recall response in memory T cells. OX40 is constitutively expressed on Tregs where it promotes Treg proliferation and immunosuppressive activity. OX40-OX40 Ligand signaling is involved in allergic airway inflammation, graft-versus-host disease, and autoimmune disease.