Substances that bind to a receptor are named ligands. Ligand-receptor interaction is the central issue in pharmacology. Each receptor in the body is recognized by an endogenous ligand (e.g. a hormone). The ligand is then called an agonist. Agonism is the production of a response by a receptor after ligand-receptor interaction. The intrinsic activity of a full agonist is defined as equal to 1. Some of the endogenous agonists or derivatives are used as drugs (e.g. growth hormone, adrenaline).
Besides these full agonists, partial agonists also exist. Partial agonists have an effect on the receptor, but the resulting response is always smaller than the effect of full agonists. The intrinsic activity lies between 0 and 1. Full and partial agonists often have similarities in their chemical structure. Antagonists are ligands that bind to a receptor, but they do not elicit a response. In fact, antagonists block the receptor from the agonists and thus inhibit the action of the agonists. The intrinsic activity of an antagonist is 0.