Model: Excess BAFF promotes activated B cells in cGVHD. (A) In patients who do not develop cGVHD, high BAFF levels are associated with increased numbers of naive B cells. This results in a normal BAFF/B-cell ratio and potentially autoreactive B cells are eliminated due to limiting amounts of BAFF. (B) In patients who develop cGVHD, high BAFF levels are associated with decreased numbers of naive B cells resulting in a high BAFF/B-cell ratio. In this setting, excess BAFF levels support alloreactive and autoreactive pre- and post-GC B cells that are most reliant on BAFF for their survival.