The CD33 CAR Detection Reagent, anti-human, Biotin, REAfinity™ has been developed for the detection of transduced T cells that are engineered to express CD33-specific chimeric antigen receptors (CAR) on the cell surface, which recognize human CD33 antigen. The CD33 CAR Detection Reagent is an antigen based detection reagent conjugated to biotin. It contains a recombinantly expressed fusion protein consisting of the human CD33 extracellular domains and a specifically mutated human IgG1 Fc region. The engineered CD33 CAR T cells can be detected via the recognition of the CD33 protein, and identified by flow cytometry via anti-biotin fluorochromes. The mutated human IgG1 Fc region of the CD33 CAR Detection Reagent abolishes their binding to Fcγ receptors. This allows for background-free analysis and eliminates the need for additional blocking steps, such as using a FcR blocking reagent.