intrinsic activity

Intrinsic activity describes how good a ligand is at switching a receptor on once it has bound — not whether it binds, but what it does after it binds. Two molecules can dock equally well into the same receptor, yet one flips the switch fully while the other only nudges it. Intrinsic activity captures that 'turning-on' ability.

It is most often expressed as a number between 0 and 1, scaled against a reference full agonist set to 1: a full agonist has an intrinsic activity of about 1 (it produces the maximal response), a partial agonist sits somewhere in between (say 0.5), a pure antagonist is 0 (it binds but does nothing), and an inverse agonist can even be negative (it pushes the receptor below its resting baseline activity). In practice the number is read from the height of a drug's maximal effect relative to the full agonist's maximum.

A subtle but real caveat: intrinsic activity defined this way is an observed, system-dependent quantity, because receptor reserve and signal amplification can make a low-efficacy ligand look like a full agonist in a responsive tissue. Pharmacologists therefore distinguish this measured intrinsic activity from the deeper, system-independent concept of efficacy. For everyday purposes, though, intrinsic activity is a handy shorthand for 'how strongly does this bound molecule activate the receptor?'