CD19 and acute lymphoblastic leukemia: To determine target‐induced activation of T cells, mock‐transduced T cells or CAR T cells were co‐cultured with CD19– U937 cell line as a negative control, autologous B cells, the CLL‐derived CD19+ Mec‐1 cell line, or the ALL‐derived CD19+ Nalm‐6 cell line at an effector‐to‐target (E:T) ratio of 1:1.