Jorg L, et al. - In this study, patients with antihistamine-resistant chronic spontaneous urticaria (CSU) were assessed to determine the mode of action of omalizumab. Omalizumab caused a rapid reduction of FcεRI receptor density on basophils. Furthermore, a cellular impact of omalizumab on basophils was suggested, as it was found that since CU-BAT using well-characterized, omalizumab-naïve donor basophils did not change during the treatment phase, autoreactive serum factors seemed to remain unaltered. FcεRI density and CU-BAT might be promising cellular-based assays for the prediction of the omalizumab response time and monitoring disease.