Although the action of H1 antagonists in the early phase of IgE-mediated, allergic skin reactions is well established, the effect of these antihistamines on the ensuing late-phase reaction has only recently been investigated. The eosinophilic late-phase reaction, sometimes associated with the tissue damage that occurs in chronic idiopathic urticaria, is important in allergic responses. Cetirizine is a new H1 antagonist with potent inhibitory action against the infiltration of eosinophils into involved tissue, thereby affecting the late-phase reaction. No other antihistamine has yet been reported to possess this antieosinophilic property. The mechanisms by which cetirizine acts against eosinophils, although not thoroughly understood, seem to involve direct action against these cells.
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