agonist
An agonist is like a key that not only fits a lock but also turns it. It binds to a receptor and switches it on, triggering the cell's response just as the body's own natural signal would.
Mechanistically, an agonist has both affinity (it sticks to the receptor) and efficacy (once bound, it activates the receptor and sets off downstream signalling). A full agonist drives the receptor to its maximal possible response. Many drugs mimic natural messengers: salbutamol is an agonist at beta-2 adrenoceptors that relaxes airway muscle, and morphine is an agonist at mu opioid receptors that relieves pain.
The size of an agonist's effect depends on how much drug is present, how tightly it binds, and how many receptors the tissue has. Because cells often have spare receptors, a full response can sometimes be reached without occupying every receptor — a point that becomes important when comparing agonists with partial agonists and antagonists.
Adrenaline is an agonist at adrenoceptors: in anaphylaxis it activates these receptors to raise blood pressure and open the airways.
Agonist mimicking a natural signal.
Whether a molecule acts as an agonist can depend on the tissue and receptor subtype; the same drug may be a full agonist at one receptor and a partial agonist or even antagonist at another.