At the time of PML (T1) the percentage of CD19+ B cells within the lymphocyte gate had increased to 44.2% (approximately four-fold SD of natalizumab-treated patients; 1,303/μL) and remained high even after plasma exchange (PE) (T2; 52.0%, 15 days post PE; 868/μL) (Figure 1C). Here, CD19 is linked to progressive multifocal leukoencephalopathy.