Serial analyses of CD19⁺CD21^low B cells from d 100 after allogeneic HCT until onset of and during the course of BOS (B) compared with patients without cGVHD (no cGVHD) (A). In both patient cohorts, CD19⁺CD21^low B cells decreased 6 mo after HCT, indicating B-cell reconstitution, because CD21^low B cells reportedly are the first B-cell population to be exported from the bone marrow into the periphery and decrease during B-cell maturation.³³  Whereas in patients without cGVHD (A) percentages of CD19⁺CD21^low B cells further decreased until 11 mo after HCT, in patients with newly diagnosed BOS (B) CD19⁺CD21^low B cells increased again and remained elevated throughout the course of BOS. Data are shown as box plots with medians and ranges. ° indicates outliers.