A histamine antagonist (commonly called an antihistamine) is a pharmaceutical drug that inhibits the action of histamine by blocking it from attaching to histamine receptors; or it may inhibit the enzymatic activity of histidine decarboxylase, catalyzing the transformation of histidine into histamine (atypical antihistaminics).
Antihistamines are commonly used for the relief of allergies caused by intolerance of proteins.
Histamines produce increased vascular permeability, causing fluid to escape from capillaries into tissues, which leads to the classic symptoms of an allergic reaction — a runny nose and watery eyes. Histamine also promotes angiogenesis.
Antihistamines suppress the histamine-induced wheal response (swelling) and flare response (vasodilation) by blocking the binding of histamine to its receptors on nerves, vascular smooth muscle, glandular cells, endothelium, and mast cells. They exert a competitive antagonism to histamines.
Itching and sneezing are suppressed by antihistamine blocking of H1-receptors on nasal sensory nerves. Antihistamines have also been used with great success in the treatment of Brown Recluse (genus Loxosceles) spider bites as well as other insect bites that cause necrosis.