Alemtuzumab exerts its clinical efficacy by its specific pattern of depletion and repopulation of different immune cell subsets. Recently, single cases of multiple sclerosis patients who developed severe exacerbation after the first alemtuzumab application, accompanied by re-appearance of peripheral B cells, were reported. Here we present a case with underlying B cell-driven multiple sclerosis that impressively improves after alemtuzumab, although peripheral B cell repopulation took place. Our detailed clinical, histopathological, imaging and immunological data suggest that alemtuzumab can act as an effective rescue treatment in highly active B cell-driven and antibody/complement-mediated multiple sclerosis type II patients.