Antihistamines

Histamine is an important mediator of immediate hypersensitivity reactions acting locally and causing smooth muscle contraction, vasodilation, increased vascular permeability, edema and inflammation.  Histamine acts through specific cellular receptors which have been categorized into four types, H1 through H4.  Antihistamines represent a class of medications that block the histamine type 1 (H1) receptors.  Importantly, antihistamines do not block or decrease the release of histamine, but rather ameliorate its local actions.  Agents that specially block other H2 receptors are generally referred to as H2 blockers rather than antihistamines.

H1 receptors are widely distributed and are particularly common on smooth muscle of the bronchi, gastrointestinal tract, uterus and large blood vessels.  H1 receptors are also found in the central nervous system.  The antihistamines are widely used to treat symptoms of allergic conditions including itching, nasal stuffiness, runny nose, teary eyes, urticaria, dizziness, nausea and cough...