Galectin-9 is a mammalian lectin with a molecular weight of 40 kD that has two conserved carbohydrate recognition domains (CRDs) and forms homodimers. It recognizes N-acetyllactosamine (Galbeta1-4GlcNAc) and T-antigen (Galbeta1-3GalNAc). Tim-3 has been reported as its ligand. Galectin-9 is expressed by lymphocytes, dendritic cells, granulocytes, eosinophils, astrocytes, endothelial cells, fibroblasts, and thymus epithelial cells. It may be retained intracellularly or transported to the cell surface whereby cleavage generates a soluble form. Galectin-9 is involved in events such as cell aggregation, adhesion, chemotaxis, and apoptosis, and is important for the regulation of the immune response. Galectin-9 induces regulatory T cells, and suppresses Th1 and Th17 responses.