Must be finite population. If not saturated, probably non-specific. The drug-receptor combination requires amplification to induce a response. This is provided by a cascade of amplification steps. Responses involving any given type of receptor are only elicited by a fairly narrow range of chemical substrates with similar structural groupings and electronic properties.
Selectivity is greater with agonists than antagonists.
Has to show binding to preferred chemicals. All drugs are expected to be found in bound or unbound state. Never assume that receptors are fixed and stable. Can change affinity or receptor number. Hydrogen bonds and Vander Waals Forces lock it into position.