CD33 and acute myeloid leukemia: Using a FACS-based indirect QIFIKIT® immunofluorescence assay we quantified the CD33 cell surface antigen density (which is comparable for monocytes and AML-MDSCs, Fig. 1g) on primary AML-blasts and on CD14+ cells and indeed observed higher (potentially competing) CD33 levels on CD14+ cells (Fig. 2c).