CAR T CELL Therapy for Pediatric, Adolescent and Young Adult Patients With CD19-Positive Leukemia

CAR19PK is a research study evaluating the use of lymphodepleting chemotherapy and chimeric antigen receptor (CAR) T cell therapy, a type of cellular therapy, for the treatment of refractory and/or relapsed leukemia. For this type of therapy, peripheral (circulating) immune cells are collected and then modified so that they can recognize an antigen, which is a particle present on the surface of a cancer cell. The CD19-CAR T cell product will be manufactured at the St. Jude Children's Research Hospital's Good Manufacturing Practice (GMP) facility. The main purpose of this study is to determine: * Evaluate different doses of fludarabine prior CAR T cell infusion * How your body processes fludarabine and cyclophosphamide, * How long the CAR T cells last in the body, * Whether or not treatment with this therapy is effective in treating people with refractory or relapsed leukemia, and * The side effects of this therapy.