Upon meeting the eligibility requirements and enrolling on study, subjects will undergo apheresis to obtain the T cells for the generation of the CD19 CAR+ T cells. In patients with a prior history of allogeneic HCT, the T cells obtained are of donor origin. The T cells are isolated from the apheresis product, the CD4 and CD8 T cells are then selected and grown separately, transduced with a lentivirus to express the CD19 CAR as well as a truncated EGFR that has no signaling capacity (noted EGFRt) and expanded in culture over a three week period. During the process of cell generation, subjects will continue to be cared for by their primary oncologist and may undergo additional treatment directed at the leukemia during this time.