• Overall response rate (ORR) [ Time Frame: Participants will be followed for the duration of the treatment, with an expected average of 3 months. ]
  Overall Response Rate (ORR) defined as Complete Response (CR) and CR with incomplete blood recovery (CRi) according to WHO criteria.
• Complete response (CR) [ Time Frame: 12 Months ]
  Duration of response defined from the time when criteria for response (CR or CRi) are met to the first documentation of relapse or progression.
• CR with incomplete blood recovery (CRi). [ Time Frame: 12 Months ]
  Duration of response defined from the time when criteria for response (CR or CRi) are met to the first documentation of relapse or progression.

• Overall survival (OS) [ Time Frame: 12 Months, 24 Months ]
  Overall Survival (OS) defined as the time from treatment to the date of death due to any cause.
• Progression free survival (PFS) [ Time Frame: 12 Months, 24 Months ]
  Progression Free Survival (PFS) defined as the time from treatment to first documentation of objective leukemic progression (date of leukaemia assessment documenting progressive disease) or to death due to any cause. Progression is assessed by BM biopsy or CSF analysis according to NCCN criteria. It is assessed at Day 30 and monthly thereafter, or earlier if clinically indicated.
• Time to next treatment (TTNT) [ Time Frame: 12 Months, 24 Months ]
  Time To Next Treatment (TTNT) defined as the end of study treatment until the institution of the next therapy.
• Percentage of adverse events [ Time Frame: 30 days ]
  Percentage of participants with adverse events

• Relapsed B Acute Lymphoblastic Leukaemia
• Refractory B Acute Lymphoblastic Leukaemia

• Biological: CD19 CAR-T CELLS
  CD19 CAR-T cells will be administered after completion of the chemotherapy.
• Drug: Cyclophosphamide
  Patients will receive lymphodepleting chemotherapies consisting Cyclophosphamide 250mg/m2/day IV(Day -5, Day -4, Day -3 ).
• Drug: Fludarabine
  Patients will receive lymphodepleting chemotherapies consisting of Fludarabine 25mg/m2/day IV (Day -5, Day -4, Day -3 ).

Inclusion Criteria:

• Patients with relapsed/refractory B-ALL in accordance with World Health Organization (WHO) classification by virtue of BM morphology, flow cytometry, cytogenetics and molecular genetics
• Age between ≥13 to ≤ 65 years
• No detectable leukaemia in the CSF (CNS-1)
• CNS leukaemia without clinically evident neurological symptoms (CNS-2; with <5 WBC per μL and cytology positive for blasts)
• Adequate organ function as defined by a creatinine clearance > 50 ml/min, serum total bilirubin < 5 times the normal value, left ventricular ejection fraction > 40%
• ECOG performance status ≤ 2
• Life expectancy > 3 months
• Post allogeneic HSCT must be ≥ Day +100 with no evidence of active GVHD and not receiving immunosuppression
• Female patients of child bearing age must have negative pregnancy test and is on highly effective contraception methods
• Male patients must use highly effective contraception methods

Exclusion Criteria:

• Patients with CNS-3 leukaemia.
• Active cancer (other than B-ALL).
• Evidence of severe lung, heart (NYHA class III/IV, arrhythmia, AV block, uncontrolled hypertension), liver, or renal failure or severe neurologic disorder.
• Presence of active autoimmune disease or atopic allergy.
• HIV serology positivity.
• Active Hepatitis B or C infection as evidenced by quantitative viral PCR assay.
• Uncontrolled sepsis
• Pregnant / nursing female.
• Ongoing prednisolone > 1mg/kg daily or equivalent.
• Chemotherapy immunotherapy in the recent 4 weeks such as allogeneic cellular therapy weeks, anti-GVHD therapy.