Antihistamines are inhibitors of histamine receptors. H1-antihistamines, inhibit competitively H1 receptors and the corresponding effects i. They do not inhibit antigen/antibodies reactions, nor histamine release, they inhibit H1 effects.
H1-antihistamines have been used for more than fifty years in treating various allergic manifestations. H1-antihistamines which penetrate into brain elicit, by inhibiting stimulant effect of histamine, drowsiness.
The drugs of the first antihistamine generation are: promethazine, alimemazine, dexchlorpheniramine, brompheniramine, buclizine, carbinoxamine and doxylamine, are sedating and elicit a drowsiness which can be awkward.
The first-genaration antihistamines have alpha adrenolytic activity which can decrease the vasoconstrictive effect of adrenaline and noradrenaline and an antimuscarinic effect with the corresponding adverse effects.
Histamine carries its message to a large number of cells by attaching to a special receptor on the cells' surfaces.
Antihistamines have a broad range of functions -- they can ease allergy symptoms, alleviate gastrointestinal complaints, prevent motion sickness and aid in sleep, to name just a few.
Antihistamines accomplish these tasks by counteracting histamine, an important protein with diverse effects in the body.
The first antihistamines, developed a few decades after the discovery of histamine, block a receptor for histamine known as the H1-receptor.
These can be thought of as the "old-fashioned" group of antihistamines.