This purpose of this study is to determine the efficacy of humanized CD19 CAR T cells (huCART19) in pediatric and young adult patients with high-risk relapsed or refractory B-cell acute lymphoblastic leukemia (B-ALL). You may be eligible to participate if you are between the ages of 1-29 years with CD19-expressing B-ALL that is refractory to chemotherapy, has relapsed in the bone marrow <36 months from diagnosis, relapsed more than once, or relapsed after bone marrow transplant or after previous CAR T cell therapy. This study involves the administration of the study drug, blood draws, assessment of leukemia response, chart reviews, and questionnaires. Reimbursement for travel may be provided. If you have questions or would like to learn more about the study, please contact the Cancer Immunotherapy Program at (215) 425-7193. The Principal Investigator is Dr. Shannon Maude and the Immunotherapy Nurse Navigator is Brooke Leibfreid, RN.