CD19 and myeloid sarcoma: Alemtuzumab targets CD52‐expressing cells and produces rapid, marked (< 200 cells/mm3) and sustained CD4 and CD8 T‐cell depletion lasting many months, whereas there is rapid repopulation of CD19+ B cells to levels above baseline.11 Importantly, this resulted in the inhibition of lesion formation and relapsing disease and alemtuzumab is currently one of the most effective licensed treatments of MS.12 The effectiveness of alemtuzumab is used to support the concept that T cells drive relapsing disease, which can be clearly shown in animal models.