CD19 and graft versus host disease: Besides CD19+CD5+CD1dhighIL-10+ Breg populations, IL-10-producing immature B cells (CD19+CD24highCD38highIL-10+) are a crucial immunomodulating cell subset in humans since they suppress effector T cell responses as well as Th1 and Th17 differentiation, promote the conversion of CD4+ T cells into regulatory T cells (Treg) and type 1 regulatory T cells (Tr1) and play additional roles during autoimmunity, HIV infection and graft-versus-host disease [21, 25–27].