Following allogenic hematopoietic stem cell transplantation (HSCT), patients with autoimmune disease or hematopoietic malignancy may develop acute or chronic graft-versus-host (GvH) disease. B lymphocytes, from the recipient as well as from the donor, have recently been implicated in the pathogenesis of such disturbances. Their deleterious effects are accounted for by other tasks B lymphocytes accomplish than the antibody production. We highlight herein some recent observations in the context of B cells in the GvH disease.