H1-receptor antagonists of the first and second generation have been used for many years to relieve symptoms arising from the action of histamine on the H1 receptor. However, allergic inflammation is more than histamine release and action. Leukocyte infiltration during late allergic manifestations was demonstrated as being a hallmark of allergic inflammation. Second-generation H1-receptor antagonists were studied in models of mast cell degranulation in vitro and of leukocyte infiltration in vivo. Analysis of the literature showed that a drug which is no more metabolized than cetirizine and fexofenadine allows a better relevance of in vitro models. Up to now, cetirizine was the only product in vivo able to reduce eosinphil infiltration in skin, nose, eyes and lungs during experimental allergenic challenges and late allergic inflammatory manifestations. Today, relevance of these properties can be seen in the recently published clinical results showing that cetirizine prevents the development of asthma in specifically sensitized infants with atopic dermatitis.