First-generation antihistamine

H₁ antagonists, also called H₁ blockers, are a class of medications that block the action of histamine at the H₁ receptor, helping relieve allergic reactions. Agents where the main therapeutic effect is mediated by negative modulation of histamine receptors are termed antihistamines; other agents may have antihistaminergic action but are not true antihistamines.

In common use, the term "antihistamine" refers only to H₁ antagonists, also known as H₁-receptor antagonists and H₁-antihistamines. It has been discovered that some H₁-antihistamines function as inverse agonists, as opposed to receptor antagonists, at the histamine H₁-receptor.

In type I hypersensitivity allergic reactions, an allergen (a type of antigen) interacts with and cross-links surface IgE antibodies on mast cells and basophils. Once the mast cell-antibody-antigen complex is formed, a complex series of events occurs that eventually leads to cell degranulation and the release of histamine (and other chemical mediators) from the mast cell or basophil. Once released, the histamine can react with local or widespread tissues through histamine receptors.

Histamine, acting on H₁-receptors, produces pruritus, vasodilation, hypotension, flushing, headache, tachycardia, bronchoconstriction, increase in vascular permeability and potentiation of pain.

...
Wikipedia