CXCL13, also called BCA-1 or BLC, is a homeostatic CXC chemokine that acts as the main chemoattractant for B lymphocytes and follicular helper T cells through exclusive binding to the G-protein-coupled receptor CXCR5, thereby guiding their migration into lymphoid follicles and organizing the microarchitecture of secondary lymphoid organs such as lymph nodes and spleen; secreted chiefly by follicular dendritic cells, stromal cells, and macrophages, it triggers downstream signaling cascades including PLC-β/PKCα, MAPK, PI3K-Akt, and NF-κB that promote chemotaxis, adhesion, and survival of B cells, and its dysregulation—manifested by elevated serum or tissue levels—has been mechanistically linked to autoimmune diseases (e.g., rheumatoid arthritis, Sjögren’s syndrome, systemic lupus erythematosus), chronic inflammatory disorders, B-cell malignancies, and neuroinflammatory conditions like multiple sclerosis, making it both a pathogenic driver and a clinically useful biomarker for disease activity and prognosis.