Abstract: The present disclosure relates generally to antibodies and binding fragments thereof that bind to anti-CD123 antibodies, chimeric antigen receptors (CARs), or antibody binding fragments. In particular, the disclosed anti-idiotype antibodies and fragments bind to anti-CD123 antibodies, CARs, or fragments thereof and comprise novel complementary determining regions (CDRs). Finally, the present disclosure relates to methods of using the disclosed antibodies and fragments thereof to expand and/or activate CD123-CAR-expressing immune cells, detecting or quantifying CD123-CARs, and isolating CD123-CAR-expressing immune cells.