The two-state model is a simple linear model to describe the interaction between a ligand and its receptor, but also the active receptor (R). The model uses an equilibrium dissociation constant to describe the interaction between ligand and receptor.  It proposes that ligand binding results in a change in receptor state from an inactive to an active state based on the receptors conformation. A receptor in its active state will ultimately elicit its biological response. It was first described by Black and Leff in 1983 as an alternative model of receptor activation. Similar to the receptor occupancy model, the theory originated from earlier work by del Castillo & Katz on observations relating to ligand-gated ion channels. In this model, agonists and inverse agonists are thought to have selective binding affinity for the pre-existing resting and active states or can induce a conformational change to a different receptor state.  Whereas antagonists have no preference in their affinity for a receptor state. The fact that receptor conformation (state) would affect binding affinity of a ligand was used to explain a mechanism of partial agonism of receptors by del Castillo & Katz in 1957 was based on their work on the action of acetylcholine at the motor endplate build on similar work by Wyman & Allen in 1951 on conformational-induced changes in hemoglobins oxygen binding affinity occurring as a result of oxygen binding. The del Castillo-Katz mechanism divorces the binding step (that can be made by agonists as well as antagonists) from the receptor activation step (that can be only exerted by agonists), describing them as two independent events.