TNFSF4 encodes OX40 ligand (OX40L), a TNF superfamily cytokine that binds to TNFRSF4 (OX40 receptor) and functions as a T-cell costimulatory molecule 1. OX40L is expressed on antigen-presenting cells, activated T cells, and other cell types; expression increases after antigen presentation and in response to pro-inflammatory signals including CD28 ligation and interferon-gamma 1. The OX40-OX40L interaction promotes T-cell survival, effector T-cell differentiation, T-cell memory formation, and increased effector cytokine production, while reducing regulatory T-cell function 12. TNFSF4 is implicated in multiple autoimmune and inflammatory conditions. Genetic variants in TNFSF4 associate with systemic lupus erythematosus and IgA nephropathy, positioning it within inflammatory signaling pathways 3. In atopic dermatitis, increased OX40L expression on antigen-presenting cells drives pathogenic T-cell proliferation across Th2, Th1, Th17, and Th22 subsets, contributing to chr1 inflammation 45. Cell-cell communication studies reveal elevated TNFSF4-TNFRSF4 interactions in thyroid-associated ophthalmopathy and esophageal squamous cell carcinoma, suggesting broader disease relevance 67. Therapeutic blockade of OX40L represents a promising strategy for treating autoimmune and inflammatory diseases.