Neutrophils are your first line of defence against infection, but they express CD52 and therefore they are depleted by alemtuzumab. This will contribute to infections post treatment, but, whilst common, the level of depletion is rarely severe and long-lasting. We have commented on this in a previous post, but you can now get a free copy of the paper.
David Baker, Gavin Giovannoni, Klaus Schmierer
Marked neutropenia: Significant but rare in people with multiple sclerosis after alemtuzumab treatment
Multiple Sclerosis and Related Disorders 18 (2017) 181–183