Histamine antagonist/Definition

A member of a class of drug that binds to cell surface receptors for histamine₁, histamine₂, and histamine₃ receptors, preventing them from activating. The receptor types have a variety of functions; histamine₁ activities include allergic reactions and nausea, histamine₂ reduce gastric acid secretion and relax smooth muscle, and histamine₃, still in research, have central nervous system effects. "Antihistamines" are most often histamine₁ blockers.