Part of the reason for CAR T cells’ astonishing success in B-cell cancers is that B cells are astonishingly easy to replace. CAR T cells are specific, yes. But they are not specific to tumor cells. They are specific to their target antigens. In the case of Yescarta (axicabtagene ciloleucel, Gilead Sciences Inc.) and Kymriah (tisagenlecleucel, Novartis AG), the first two clinically approved T cells, that target is CD19, which is expressed on B-cell precursors. And when it is successful, the treatment leaves patients without any B cells at all.