On-target, off-tumor side effects of CD19-targeted CAR–T-cell therapy. (A) Depiction of a CD19-targeted CAR-T cell that has both “on-target, on-tumor” and “on-target, off-tumor” activity. (B) The lineage of B cells from early to fully differentiated cells depicting expression of the CD19 cell surface antigen on pre-B cells, naïve B cells, and memory B cells but not on antibody-producing plasma cells.