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The second-generation H receptors and cholinergic receptors.This selectivity significantly reduces the occurrence of adverse drug reactions, such as sedation, while still providing effective relief of allergic conditions.
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 type I hypersensitivity allergic reactions, an allergen (a type of antigen) interacts with and cross-links surface Ig E antibodies on mast cells and basophils.
H-receptor are used to treat gastric acid conditions (e.g., peptic ulcers and acid reflux).
Histamine receptors exhibit constitutive activity, so antihistamines can function as either a neutral receptor antagonist or an inverse agonist at histamine receptors.
The isomerism is a significant factor in the activity of the agents in this group.
E-triprolidine, for example, is 1000-fold more potent than Z-triprolidine.
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.-antihistamines can be administered topically (through the skin, nose, or eyes) or systemically, based on the nature of the allergic condition.
This lack of receptor selectivity is the basis of the poor tolerability profile of some of these agents, especially when compared with the second-generation HDiphenhydramine was the prototypical agent in this group.
Significant anticholinergic adverse effects, as well as sedation, are observed in this group but the incidence of gastrointestinal adverse effects is relatively low.
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.