Histamine has an important role as a chemical messenger in physiologic responses,
neurotransmission, allergic inflammation, and immunomodulation by way of the H1-receptor.
Most H1-antihistamines, which are useful in treating these effects, possess similar efficacy in
allergic rhinoconjunctivitis and chronic urticaria. However, there are clinically relevant
differences among them in their pharmacology and safety profiles.