CD19 and graft versus host disease: Elevated levels of CD19+CD21low B cells in the peripheral blood of allo-HSCT recipients with acute leukaemia have been proposed as a biomarker of the development of GvHD in the lung [108], and rituximab mediated prophylactic pre-transplant B cell depletion reduces the incidence of GvHD in allo-HSCT recipients with CLL and mantle cell lymphoma [109].