ligand /lig″ənd, lī″gənd/ [L, ligare, to bind] , 1. a molecule, ion, or group bound to the central metal atom of a chemical compound, such as the oxygen molecule in oxyhemoglobin, which is bound to the central iron atom. 2. an organic molecule attached to a specific site on a cell surface or to a tracer element. The binding is reversible in a competitive binding assay. It may be the analyte or a cross-reactant. Examples include vitamin B12, a ligand with intrinsic factor as the binding protein, and various antigens, which are ligands with antibody-binding proteins.
