(A) BAFF and CD19⁺CD21^low B cells at onset of BOS compared with patients without cGVHD (no cGVHD). Patients with newly diagnosed BOS have significantly higher BAFF levels and percentages of CD19⁺CD21^low B cells compared with the no cGVHD cohort. Data are shown as box plots with medians and ranges, and P values are indicated. (B) Receiver operating characteristic curves for percentages of CD19⁺CD21^low B cells (dotted line), the ratio of BAFF/CD19⁺ B cells (dotted and dashed line), and BAFF (solid line). Individual receiver operating characteristic curves are shown.