Fexofenadine

The H1 histamine receptor is responsible for mediating hypersensitivity and allergic reactions. Exposure to an allergen results in degranulation of mast cells and basophils, which then release histamine and other inflammatory mediators. Histamine binds to, and activates, H1 receptors, which results in the further release of pro-inflammatory cytokines, such as interleukins, from basophils and mast cells. These downstream effects of histamine binding are responsible for a wide variety of allergic symptoms, such as pruritus, rhinorrhea, and watery eyes.

Fexofenadine is considered an “inverse agonist” of the H1 receptor because it binds to and stabilizes the inactive form of the receptor, preventing its activation and subsequent downstream effects. It has a potent and selective affinity for H1 receptors, and there is no evidence that it carries antidopaminergic, antiserotonergic, anticholinergic, sedative, or adrenergic blocking activity. Fexofenadine does not cross the blood-brain barrier and thus is unlikely to cause significant CNS effects.