The binding of histamine to H1 receptors may result in an increase in the capacity of antigen-presenting cells and in the release of histamine and other mediators from mast cells and basophils,which plays an important role in the pathogenesis of urticaria and other allergic diseases.First-generation H1-antihistamines,which have lipophilicity and low molecular weight,can penetrate blood-brain barrier easily and induce a number of side effects,especially on alertness and consciousness.By contrast,second-generation H1-antihistamines have higher molecular weight,specificity and affinity for receptor,stronger antihistamine activity,and superior safety.International treatment guidelines all recommend second-generation H1-antihistamines as first-choice therapy for chronic urticaria,which permits a dose increase up to 4 times the standard dose when needed with a good security.H3-and H4-antihistamines are under clinical investigations,and are expected to be used for the treatment of allergic diseases and pruritus.