agonist
An agonist is like a key that not only fits a lock but turns it. It binds to a receptor and switches it on, triggering the cell's response — the same job the body's own natural messenger would do.
More precisely, an agonist has two properties: affinity, which lets it bind the receptor, and intrinsic activity (or efficacy), which lets it stabilize the receptor's active shape so downstream signaling fires. A full agonist drives the maximal response the receptor system can produce; a partial agonist binds well but pushes the response only partway.
Many medicines are agonists that mimic a natural signal. Salbutamol activates β2-adrenergic receptors to relax airway muscle in asthma; morphine activates opioid receptors to blunt pain. Because agonists actively turn receptors on, prolonged exposure often causes the cell to desensitize or down-regulate those receptors, which underlies tolerance.
Salbutamol is a β2-adrenergic agonist: it binds the receptor and activates it, relaxing bronchial smooth muscle to open the airways during an asthma attack.
An agonist mimics a natural signal to switch a receptor on.
An agonist must possess both affinity (to bind) and efficacy (to activate). A molecule with affinity but zero efficacy is an antagonist, not an agonist.