Alemtuzumab is a potent monoclonal CD52 antibody used to treat patients with multiple sclerosis (MS). However, recent literature reports have described paradoxical activation of B cell–mediated disease within 1 year of the first cycle of alemtuzumab. We raise awareness that severe B cell–mediated disease activation could develop, even after two cycles of alemtuzumab, in some vulnerable MS patients; therefore, individualized therapeutic strategies should be considered in clinical practice. We also propose that a novel regulatory B-cell subset may be a candidate for a predictive biomarker of disease activation in MS patients treated with alemtuzumab.
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