Histamine influence immunoregulation of the acute and chronic inflammatory response through 4 different types of receptors, called H1, H2, H3, and H4. Drugs classified in the first generation of antihistamines (“classical” antihistamines) are classified as first generation, include chlorpheniramine, diphenhydramine, promethazine, and hydroxyzine. They are sedating and necessitate multiple daily doses because of relatively short half-lives.
The second generation antihistaminincs are relatively nonsedating, include Loratadine, Cetirizine, Fexofenadine(Allegra) etc. These have a poor penetration of the blood brain barrier and thereby less likely to be sedating than first generation drugs with longer half-lives, permitting once- or twice-daily dosing.