A histamine antagonist is a drug that binds to certain receptors for the chemical messenger histamine and prevents the effects of this chemical in the body. Some of these medications, also called antihistamines, diminish the vasodilation responsible for common allergy symptoms like runny nose and swelling. The early drugs in this series also had broad effects on the nervous system including sedation, but later ones were more specific. Another group of histamine antagonists prevents the secretion of stomach acid and is used to treat indigestion.
Histamine is a modified form of the amino acid histidine that increases the permeability of blood vessels to the cells of the immune system, allowing these to reach sites of infection and injury. A histamine antagonist is a chemical binding to H1 or H2 receptors for histamine, and blocks their actions on the body. When it binds to H1 receptors of the respiratory epithelium, histamine mediates the nasal inflammation and breathing difficulties caused by allergies. Another class of histamine receptors, H2, initiate the release of digestive acid in the stomach by the parietal cells.
Allergic reactions are hypersensitivities, triggering an immune cascade that releases histamine and mediates vasodilation. The histamine antagonist drugs called antihistamines bind to the H1 receptors located in respiratory epithelium. This prevents the receptors from causing common symptoms of the allergic response.
The runny nose and eye tearing associated with allergies are the result of the histamine-induced vasodilation blocked by antihistamines. Sneezing is triggered in part by H1 receptors in nasal nerves, and antihistamine action prevents activation of these. While histamine antagonists block the chemical's actions, they do not prevent it from being manufactured by the body.
The first-generation antihistamines, like diphenhydramine, had sedative side effects, often causing drowsiness. Sometimes they are used in over-the-counter sleep aids and mild sedatives to exploit this effect. Those effects occurred because these drugs were not very selective for the H1 receptor, and also bound to sites with inhibitory effects on the nervous system, such as inducing sleepiness. Serotonin receptors were one such site of antihistamine action. The second-generation antihistamines, like loratidine, were designed to be a more specific H1 histamine antagonist, limiting their side effects as compared to earlier drugs.
Gastric acid is secreted by the parietal cells of the stomach in response to chemical messengers including histamine. Excessive acid production is a cause of indigestion or heartburn. An early class of antacid medications work as a histamine antagonist, competing with histamine to bind to the H2 receptors and to prevent the parietal cells from releasing gastric acid. Cimetidine is an example of these H2 blockers used to treat indigestion, and sometimes to relieve peptic ulcer symptoms. Another class of drugs used to treat heartburn, the proton-pump inhibitors, has proved more effective than the histamine antagonist.