receptor

A popular and generally accepted hypothesis that appears to explain many pharmacodynamic phenomena holds that specialized protein molecules on the surfaces of cells provide a "fit" for an intrinsic molecule (such as a hormone or neurotransmitter) or a drug such that when that molecule occupies (binds to) that area, it leads to a biochemical or physiologic response. This idea is often compared to the operation of a lock (receptor) by a key (ligand).