Interestingly, relative numbers of CD19+CD21low B-cells were significantly elevated both at onset of BOS as well as in patients with long-lasting BOS not responding to immunosuppressive treatment confirming a role of CD19+CD21low B-cells as cellular biomarkers for objective diagnosis of lung involvement as well as continued cGvHD activity during the course of therapy. Here, CD19 is linked to Buschke-Ollendorff syndrome.