What is epinephrine and why is it the first-line treatment for anaphylaxis?

Epinephrine is an adrenergic agonist that is the only first-line treatment for anaphylaxis because it rapidly counteracts all the life-threatening features of a severe allergic reaction. It constricts blood vessels to reverse hypotension and shock through alpha-1 stimulation, relaxes bronchial smooth muscle to open airways through beta-2 stimulation, increases heart rate and contractility through beta-1 stimulation, and suppresses further mediator release from mast cells. Intramuscular injection in the anterolateral thigh is the preferred route.