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Antihistamines and Histamine
Antihistamines are a group of synthetic drugs that can inhibit various actions of histamine .
They have some chemical resemblance to histamine and act as antagonists by competing with histamine for occupation of its receptor sites , thereby preventing histamine from eliciting its usual responses .
They are helpful therapeutically in preventing rather than in reversing histamine actions .
Antihistamines (such as diphenhydramine and chlorpheniramine ) have been available since 1945 , although subsequently they have been designated more precisely as H1-receptor histamine antagonists or H1-receptor blockers .
The H1 antihistamines are used to suppress or alleviate the symptoms in various allergic conditions ; they do so by competing with the released histamine for occupation of its H1 receptors .
In general , the H1 antihistamines tend to be more successful in controlling acute than chronic conditions ; thus , they are most useful at the beginning of the hay-fever season , when the allergens are present in low concentration , but in perennial vasomotor rhinitis ( nonseasonal , nonallergic inflammation of the mucous membranes of the nose brought on by environmental or emotional stimuli ) they are only of limited value .
They are not usually effective in treating asthma , indicating that in this condition histamine is not the main agent producing the symptoms .
Certain allergic skin reactions respond favourably to H1 antihistamines , which are particularly effective for treatment of acute urticarial rashes of the skin and the itch and swelling of insect bites .
The H1 antihistamines are relatively free from serious side effects .
The older antihistamines bind strongly to H1 receptors in the brain but also bind to muscarinic receptors in the brain , and this action may contribute to their beneficial effect .
In the 1970s a new class of synthetic drugs was invented that blocked the action of histamine at its H2 receptors ( see the section Digestive system drugs) .
These drugs were shown to be extremely effective in antagonizing the action of histamine in stimulating acid secretion and in blocking other stimulants of acid secretion , including the hormone gastrin and food .
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