B-cell homeostasis in chronic GVHD. The top part of the figure schematizes the physiologic B-cell maturation from bone marrow to the peripheral blood of main B-cell subsets. BCR indicates B-cell receptor; Il7-R, interleukin 7 receptor; T1, transitional 1 B cell; T2, transitional 2 B cell; Cs, class switch; and GC, germinal center. The bottom part of the figure schematizes the current knowledge of these B-cell subsets in the pathophysiology of chronic GVHD. Breg indicates B regulatory cells; Treg, T regulatory cells; Tconv, conventional T cells; and BAFF, B cell activation factor.