TL1A, or Tumor Necrosis Factor Superfamily Member 15 (TNFSF15), also known as TL1 or VEGI, is a type II transmembrane protein belonging to the TNF ligand superfamily. It functions as a key regulator of both innate and adaptive immune responses. While expressed primarily in glandular, squamous, and specialized epithelial cells, TL1A is also found in immune cells such as macrophages and dendritic cells. Its C-terminal extracellular domain contains a TNF homology domain (THD), forming a homotrimeric structure. The functional receptor for TL1A is Death Receptor 3 (DR3, TNFRSF25), a type I transmembrane protein. TL1A's signaling pathway is modulated by decoy receptor 3 (DcR3), which competes with DR3 for ligand binding. Upon binding to DR3, TL1A can activate pro-inflammatory signaling cascades (NF-κB, MAPK, PI3K) or, alternatively, trigger TRADD dissociation and FADD-mediated apoptosis via caspase-8 and caspase-3 activation. This dual signaling capacity makes TL1A a target of interest for the treatment of inflammatory bowel disease (IBD), allergic diseases, and autoimmune conditions.