Ligand

A ligand is any molecule that binds to a specific site on a receptor, enzyme or other protein, whether it activates, blocks or merely occupies that site.

The word comes from the Latin for binding and is deliberately neutral about outcome. Hormones, neurotransmitters, drugs and endocannabinoids are all ligands for their respective receptors. Pharmacologists describe a ligand by its affinity, how tightly it binds, and its efficacy, how much response it produces once bound; the combinations of these two properties give the categories agonist, partial agonist, antagonist and inverse agonist. An endogenous ligand is one the body makes itself, such as anandamide, while an exogenous ligand comes from outside, such as THC.

A receptor whose natural ligand has not been identified is called an orphan, a label still applied to GPR18 and GPR55. In cannabinoid science, CBD is a ligand for many proteins, including TRPV1 channels, 5-HT1A receptors and PPAR-gamma, typically with modest affinity. Which of those interactions matters in a living body remains a subject of ongoing study.