Galectin-1 is a 14 kD lectin widely expressed in mammals and produced by vascular, interstitial, epithelial, and immune cells. This protein binds specifically to galactoside derivatives of glycoproteins on cell membranes. A great number of membrane glycoconjugates have been identified as binding partners of Galectin-1 such as β1 integrins, CD2, CD3, CD4, CD43, CD45, and GM1 ganglioside. Galectin-1 also binds to a number of extracellular-matrix components, such as laminin and fibronectin in a dose-dependent and β-galactoside-dependent manner. Because Galectin-1 is a divalent lectin, it can also form complexes with receptor partners participating in cell-matrix recognition. Thus, Galectin-1 is involved in a variety of biological activities such as cell-matrix interactions, immune responses, inflammatory responses, tumorigenesis, and apoptosis of both tumor and immune cells. In addition, Galectin-1 is thought to play a role in creating immune tolerance in pregnancy. Galectin-1 induces the differentiation of dendritic cells towards a phenotype which dampens T helper 1 cells and T helper 17 cells and dampens inflammation via IL-10 and IL-27. It also plays a role in the formation and expression of HLA-G in the syncytium. Galectin-1 may act as an autocrine negative growth factor that regulates cell proliferation.