Trial: Prospective, single-center, real‑world observational study in relapsed/refractory large B‑cell lymphoma assessing predictors of response to CD19 CAR‑T. Drug/intervention: CD19-directed CAR‑T cell therapy (autologous, genetically engineered T‑cell immunotherapy). Mechanism: Patient T cells are modified to express a chimeric antigen receptor that binds CD19 on B cells, engagement activates T‑cell signaling via CD3ζ with costimulation (e.g., CD28 or 4‑1BB), driving T‑cell activation, proliferation, cytokine release, and perforin/granzyme‑mediated cytotoxic killing of malignant B cells, often leading to B‑cell aplasia. Cells/pathways targeted: CD19+ B‑cell lymphoma cells, CAR‑mediated T‑cell receptor signaling and costimulatory pathways, immune effector and tumor microenvironment interactions. The study correlates baseline clinical/tumor features, CAR‑T subset dynamics, and immune microenvironment with outcomes.