Cetirizine hydrochloride is a foundational second-generation antihistamine that exists as a racemic mixture of its R- and S-enantiomers. It binds selectively to peripheral H1 receptors across the respiratory and dermal tracts, fully suppressing histamine-mediated capillary dilation, itching, and tissue edema. Uniquely, at therapeutic levels, it also inhibits the late-stage migration of inflammatory eosinophils to allergy sites, providing dual protection against early-phase and late-phase allergic cascades while showing limited penetration through the blood-brain barrier.