The CD19 CAR FMC63 Idiotype Antibody, REAfinity™ has been developed for the detection of genetically modified T cells that are engineered to express a chimeric antigen receptor (CAR) comprising a single chain variable fragment (scFv), which derives from the mouse anti-human CD19 antibody clone FMC63. The CD19 CAR FMC63 Idiotype Antibodies, REA1297 and REA1298, are anti-idiotype antibodies that recognize the FMC63 scFv and feature a specifically mutated human IgG1 Fc region. The mutated human IgG1 Fc region of the CD19 CAR FMC63 Idiotype Antibody abolishes its binding to Fcγ receptors, which allows for background-free analysis and eliminates the need for additional blocking steps, such as using a FcR blocking reagent.