OX40 Ligand (OX40L), a type II transmembrane protein primarily expressed on antigen-presenting cells (such as dendritic cells, B cells, and activated macrophages), as well as on endothelial cells and some non-hematopoietic cells. Its receptor, OX40 (CD134), is expressed on activated T cells. The OX40-OX40L interaction provides a potent co-stimulatory signal to T cells, promoting their survival, proliferation, cytokine production, and the generation of memory T cells. In the context of allergic and autoimmune inflammation, this pathway is critical for driving and sustaining pathogenic T helper 2 (Th2) and other effector T cell responses.