Intervention: Tisagenlecleucel (Kymriah), an autologous anti‑CD19 chimeric antigen receptor (CAR) T‑cell therapy for relapsed/refractory B‑cell ALL. Mechanism of action: Patient T cells are engineered to express a CAR that recognizes CD19 and signals via CD3ζ with 4‑1BB costimulation, inducing T‑cell activation, proliferation, cytokine release, and cytotoxic killing (perforin/granzyme), resulting in clearance of CD19+ leukemic blasts and B‑cell aplasia. Targets/pathways: CD19 on B‑lineage cells, T‑cell activation through CD3ζ and 4‑1BB costimulatory pathway.