ligand

A ligand is any molecule that binds to a specific site on a target — think of it as the 'key' for the receptor 'lock'. The word comes from the Latin for 'to bind' and says nothing about what happens after binding: a ligand may switch the target on, switch it off, or simply occupy the site.

In pharmacology, ligands include the body's own hormones and neurotransmitters as well as the drugs we design to imitate or block them. What makes a molecule a ligand for a particular receptor is selective, reversible binding driven by complementary shape and weak chemical forces such as hydrogen bonds, ionic attraction and hydrophobic contacts — not the formation of permanent covalent bonds (though a few drugs do bind covalently).

Ligands are classified by their functional effect: agonists activate the receptor, antagonists block it, and modulators fine-tune its response. A radioactively or fluorescently labelled ligand can also be used purely as a tool, to count receptors or measure how tightly drugs bind in a laboratory binding assay.