Levocetirizine selectively and reversibly binds H1 receptors on endothelial and sensory nerve cells, stabilizing the inactive receptor conformation and blocking histamine-induced vasodilation, increased vascular permeability, and pruritus. The drug exhibits two- to threefold higher receptor affinity than racemic cetirizine, achieving rapid symptom relief at half the dose. Because it is a zwitterion at physiologic pH and is actively effluxed by P-glycoprotein at the blood–brain barrier, central nervous system exposure remains low, limiting sedation at recommended doses.