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Antihistamine Drugs — Molecular Selectivity and H1–H4 Receptors
Antihistamines are classified by their ability to block histamine receptors, primarily the H1 receptor responsible for allergic responses. By 2026, the clinical preference has shifted entirely to second and third-generation agents (e.g., Fexofenadine, Desloratadine) due to their molecular size and lipophilicity, which prevents them from crossing the blood-brain barrier.
Pharmacological nuances include:
Peripheral Selectivity: Unlike first-generation drugs (like Diphenhydramine), modern agents do not cause central nervous system sedation or anticholinergic effects like dry mouth and urinary retention.
H3 and H4 Receptor Research: Scientists are exploring H3 receptors (which regulate neurotransmitters) and H4 receptors (which modulate immune cell chemotaxis) as targets for treating chronic pain and autoimmune inflammation.
Intranasal Formulations: Drugs like Azelastine provide localized H1 blockade with high concentration at the nasal mucosa, offering faster relief for allergic rhinitis than oral systemic tablets.