CD19 and acute lymphoblastic leukemia: To investigate the long term efficacy and side effects of a donor-derived CD19 chimeric antigen receptor(CAR)T-cell(HI19α-4-1BB-ζ CAR-T)therapy in the treatment of patients with relapsed B-cell acute lymphoblastic leukemia(B-ALL)after allogeneic hematopoietic stem cell transplantation(allo-HSCT).