H1 antagonist

H1 antagonists, also called H1 blockers, are a class of medications that block the action of histamine at the H1 receptor, helping to relieve allergic reactions. Agents where the main therapeutic effect is mediated by negative modulation of histamine receptors are termed antihistamines; other agents may have antihistaminergic action but are not true antihistamines. In common use, the term 'antihistamine' refers only to H1-antihistamines. Virtually all H1-antihistamines function as inverse agonists at the histamine H1-receptor, as opposed to neutral antagonists, as was previously believed. In type I hypersensitivity allergic reactions, an allergen (a type of antigen) interacts with and cross-links surface IgE antibodies on mast cells and basophils. Once the allergen cross-links Immunoglobulin E, tyrosine kinases rapidly signal into the cell, leading to cell degranulation and the release of histamine (and other chemical mediators) from the mast cell or basophil. Once released, the histamine can react with local or widespread tissues through histamine receptors. Histamine, acting on H1-receptors, produces pruritus, vasodilation, hypotension, flushing, headache, bradycardia, bronchoconstriction, increase in vascular permeability and potentiation of pain. While H1-antihistamines help against these effects, they work only if taken before contact with the allergen. In severe allergies, such as anaphylaxis or angioedema, these effects may be of life-threatening severity. Additional administration of epinephrine, often in the form of an autoinjector, is required by people with such hypersensitivities. H1-antihistamines are clinically used in the treatment of histamine-mediated allergic conditions. These indications may include: H1-antihistamines can be administered topically (through the skin, nose, or eyes) or systemically, based on the nature of the allergic condition. The authors of the American College of Chest Physicians Updates on Cough Guidelines (2006) recommend that, for cough associated with the common cold, first-generation antihistamine-decongestants are more effective than newer, non-sedating antihistamines. First-generation antihistamines include diphenhydramine (Benadryl), carbinoxamine (Clistin), clemastine (Tavist), chlorpheniramine (Chlor-Trimeton), andbrompheniramine (Dimetane). However, a 1955 study of 'antihistaminic drugs for colds,' carried out by the U.S. Army Medical Corps, reported that 'there was no significant difference in the proportion of cures reported by patients receiving oral antihistaminic drugs and those receiving oral placebos. Furthermore, essentially the same proportion of patients reported no benefit from either type of treatment.'