Oda Y, Fukunaga A, Washio K, et al. - This study was undertaken to evaluate basophil responsiveness through FcεRI [high-affinity IgE (immunoglobulin E) receptor] stimulation, as well as FcεRI expression and IgE binding on blood basophils from chronic spontaneous urticaria (CSU) patients prior to as well as following omalizumab therapy and its likely link with the clinical response. This study involved 34 CSU patients managed with omalizumab. Participants were grouped as fast responders (FRs) (n = 20) and non or slow responders (N/SRs) (n = 14). Posttreatment with omalizumab, raised CD203c responsiveness was evident in FRs vs N/SRs. This improvement of basophil responsiveness, via FcεRI stimulation in FRs, was not found in peripheral blood basophils pre-incubated with omalizumab in vitro, indicating that omalizumab has no direct impact on circulating pre-existing abnormal basophils. In this study, raised basophil responsiveness via FcεRI following omalizumab therapy was shown to be related to the therapeutic impact and mechanism of action of omalizumab.