Antihistamines in Animals

Antagonists that selectively block specific histamine receptors have been developed. H₁ antagonists block the actions of histamine responsible for increased capillary permeability and wheal and edema formation. However, because histamine is only one component of an incredibly complex inflammatory cascade, antihistamines have very weak anti-inflammatory activity. H₁ antihistamines may be useful to treat immediate hypersensitivity reactions such as anaphylaxis by blocking bronchoconstriction and vasodilation. H₁ antagonists may be less effective to treat allergic inflammatory diseases, such as atopy, primarily because mediators other than histamine play important roles in such conditions. H₂ (now classified as inverse agonists of the H₂ receptor, such as cimetidine and ranitidine) antagonists are routinely used to block the gastric secretory effects of histamine and have limited anti-inflammatory effects.