Understanding Antihistamines: Relief Mechanism in Allergy Reactions

SCENARIO 3 Itchy A 20 year old student complained of an itchy nose, watery eyes and sneezing after cleaning his boarding room. Symptoms are accompanied by bumps all over the body. This is often felt every time you inhale dust. It is known that this reaction occurs due to the release of chemical mediators by certain immune cells due to an e Explain the mechanism of antihistamine drugs in type I hypersensitivity reactions LO 4 Tutor Instruction Questions: How can antihistamine drugs relieve allergy symptoms? Therapy for type I hypersensitivity disease is aimed at inhibiting mast cell degranulation, neutralizing the effects of mast cell mediators, and reducing inflammation. Antihistamines are given to block the action of histamine on blood vessels and smooth muscle. The effects of histamine released in the body can be reduced in several ways. Physiological antagonists, especially epinephrine, have effects on smooth muscle that are opposite to those of histamine, but they act on different receptors. This is clinically important because epinephrine injection can be lifesaving in systemic anaphylaxis and in other conditions where massive release of histamine and mediators occurs. Release inhibitor reduces mast cell degranulation which is the result of the immunological response by antigen-IgE interaction. Beta2-adrenoceptor agonists also appear capable of reducing histamine release. For more than 70 years, compounds that competitively antagonize many of the actions of histamine on smooth muscle have been available. The development of selective H2 receptor antagonists has led to more effective therapies for peptic disease. Selective H3 and H4 antagonists are not yet available for clinical use. H1 receptor antagonist H1 antagonists are divided into first generation and second generation agents. These groups are distinguished by the relatively strong sedative effect of most first generation drugs. First generation agents are also more likely to block autonomic receptors. Second-generation H1 blockers have less of a sedative effect, partly because of reduced distribution to the central nervous system. Both neutral H1 antagonists and inverse H1 agonists will reduce or block the action of histamine by competitively binding to H1 receptors. They have negligible potency at H2 receptors and little at H3 receptors. For example, histamine-induced contractions of bronchiolar or gastrointestinal smooth muscle can be completely blocked by this drug, but stimulation of gastric acid secretion and cardiac stimulation are unaffected. First generation H1 receptor antagonists have multiple effects, in addition to blockade of histamine action. This large number of effects may result from general structural similarity to the structure of drugs that have effects on the muscarinic cholinoceptor, αadrenoceptor, serotonin, and local anesthetic receptor sites. Some of the effects of this drug have therapeutic value but some of the effects are undesirable. Effects of H1 antagonists: 1. Sleepy The general effect of first generation H1 antagonists is sedation, but the intensity of the effectthis varies. The effects of this drug resemble those of some antimuscarinic drugs. H1 antagonist the second generation has little or no sedative or stimulant effect. Drug- These drugs also have far fewer autonomic effects than the next generation antihistamines First. 2. Anti-vomiting and anti-nausea The first few generations of H1 antagonists had significant activity inprevent motion sickness. Some are less effective against hangoversjourney. A specific H1 antagonist, doxylamine, is used as an anti-emetic inpregnancy. 3. Antiparkinsonian Some H1 antagonists, especially diphenhydramine, have significant effectssuppresses extrapyramidal symptoms associated with antipsychotic drugs. This medicinegiven parenterally for acute dystonic reactions to antipsychotics. 4. Antimuscarinic Many first generation agents, especially from the ethanolamine and subgroups ethylenediamine, has significant atropine-like effects on muscarinic receptorsperipheral. 5. Adrenoceptor blockade This can cause orthostatic hypotension 6. Serotonin blockade 7. Local anesthesia H2 receptor antagonist The development of H2 receptor antagonists was based on the observation that H1 antagonists has no effect on acid secretion in the stomach. Molecular manipulation of histamine molecules produces drugs that block acid secretion and have no effect on antagonists H1. Like other histamine receptors, the H2 receptor displays constitutive activity, and some H2 blockers are inverse agonists. High prevalence of ulcer disease pepticum created interest in exploring the therapeutic potential of H2 receptor antagonists after histamine, Serotonin, & Ergot Alkaloids were first discovered. Although this agent not the most effective, their ability to reduce gastric acid secretion with very low toxicity, making them very popular in use.