The complete remission rate reached up to 80% in CD19‐positive B cell acute lymphoblastic leukemia (B‐ALL) patients treated with CD19 CAR T.4However, accompanying adverse effects such as severe cytokine release syndrome (CRS), neurotoxicity, and graft‐versus‐host disease (GVHD) have restricted the clinical application of CAR T therapy.5 Here, CD19 is linked to graft versus host disease.