Most drugs act by being either agonists or antagonists at receptors that respond to chemical messengers such as neurotransmitters. An agonist binds to the receptor and produces an effect within the cell. An antagonist may bind to the same receptor, but does not produce a response, instead it blocks that receptor to a natural agonist. A partial agonist can produce an effect within a cell that is not maximal and then block the receptor to a full agonist. Antagonism may be competitive and reversed by higher concentrations of agonist. Insurmountable antagonists bind strongly to the receptor and are not reversed by additional agonist. Pharmacological receptors can be divided into four superfamilies: ligand-gated ion channels, G-protein coupled receptor, direct enzyme-linked receptors, and intracellular receptors affecting gene transcription.