Antihistamine
The vast majority of marketed H-antihistamines are receptor antagonists and only a minority of marketed compounds are inverse agonists at the receptor. Clinically, H-antihistamines are used to treat allergic reactions and mast cell-related disorders. Sedation is a common side effect of H-antihistamines that readily cross the blood–brain barrier; some of these drugs, such as diphenhydramine and doxylamine, are therefore used to treat insomnia. H-antihistamines can also reduce inflammation, since the expression of NF-κB, the transcription factor the regulates inflammatory processes, is promoted by both the receptor's constitutive activity and agonist (i.e., histamine) binding at the H receptor.
Histidine decarboxylase
Antihistamines are a class of medications designed to reduce unwanted effects of histamine in the body. Typical antihistamines block specific histamine receptors, depending on what physiological purpose they serve. For example, diphenhydramine (Benadryl™), targets and inhibits the H1 histamine receptor to relieve symptoms of allergic reactions. Inhibitors of histidine decarboxylase can conceivably be used as atypical antihistamines. Tritoqualine, as well as various catechins, such as epigallocatechin-3-gallate, a major component of green tea, have been shown to target HDC and histamine-producing cells, reducing histamine levels and providing anti-inflammatory, anti-tumoral, and anti-angiogenic effects.