Term used in biochemistry for a molecule that can bind to a target protein, for example a receptor. The binding of the ligand is usually reversible and is made possible in particular by ion bonds, hydrogen bonds and Van der Waals forces. The affinity of a ligand to the target protein varies. In contrast to substrates, ligands are not converted by the target molecule, but can influence its three-dimensional structure and function. Ligands that activate a receptor are called agonists, ligands that inactivate a receptor are called antagonists.