Several of the older drugs, called first-generation antihistamines, bind non-selectively to H receptors in the central nervous system as well as to peripheral receptors, and can produce sedation, inhibition of nausea and vomiting, and reduction of motion sickness.
The second-generation antihistamines bind only to peripheral HThe first-generation antihistamines may be divided into several chemical classes.
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 H 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.You will be able to get a quick price and instant permission to reuse the content in many different ways.-receptor are used to treat allergic reactions in the nose (e.g., itching, runny nose, and sneezing) as well as for insomnia. For example, histamine-induced contraction of bronchiolar or gastrointestinal smooth muscle can be completely blocked by these agents, but the effects on gastric acid secretion and the heart are unmodified. The effect resembles that of some antimuscarinic drugs and is considered very unlike the disinhibited sedation produced by sedative-hypnotic drugs. At ordinary dosages, children occasionally (and adults rarely) manifest excitation rather than sedation.
At very high toxic dose levels, marked stimulation, agitation, and even convulsions may precede coma.
Once the mast cell-antibody-antigen complex is formed, a complex series of events occurs that eventually leads 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.
Many of the first-generation agents, especially those of the ethanolamine and ethylenediamine subgroups, have significant atropine-like effects on peripheral muscarinic receptors.
This action may be responsible for some of the (uncertain) benefits reported for nonallergic rhinorrhea but may also cause urinary retention and blurred vision. This drug is promoted as an antiserotonin agent and is discussed with that drug group.
They will also reduce histamineinduced secretions, including excessive tears and salivation.