H1-Antihistamines for Allergic Diseases: Old Aged but Not Old-Fashioned Drugs

Histamine, a chemical messenger synthesized from the amino acid histidine by L-histidine decarboxylase, plays an important role in the system of immunoregulation and in acute and chronic allergic inflammation. This chemical messenger acts on 4 subtypes of receptors, H1, H2, H3 and H4, heptaelical transmembrane molecules that activate specific G-proteins. Its action on H1-receptors potentially leads to increasing antigen-presenting cell capacity, increasing release of histamine and other mediators from mast cells and basophils, upregulating Th1 priming, Th1 proliferation, interferon (IFN) gamma production, cellular adhesion molecule expression and chemotaxis of eosinophils and neutrophils. The activation of specific receptors placed on the membranes of different cellular jambs (mast cells, endothelial cells, sensory nerve fibres, bronchial smooth muscle) leads to different biological actions: vasodilatation, increased vascular permeability, pruritus, smooth muscle contraction, coronary spasm, sleep-wake rhythm regulation. H1-antihistamines are functionally classified into two groups: older "first generation H1-antihistamines" and newer "second generation H1-antihistamines".