Flow cytometric analysis of red cell D antigen expression of patients with spontaneous mixed RhD phenotype. Representative immunofluorescence histograms of RBCs of a D-positive (CcDdee) control and of patients 1 and 6 indirectly stained with 2 different monoclonal anti-D antibodies (P3x241 and MS26) are shown. Note D-positive and D-negative RBC subpopulations in patient 1. Similar results were obtained with further 9 monoclonal anti-D antibodies, and samples of patients 2, 3, 4, 5, 7, and 9. In comparison, a proportion of RBCs of patient 6 was reactive only with some but not all anti-D clones (see nonreactivity with clone MS26) indicating the absence of certain D epitopes (EpD) from his D-positive cells; the D epitope expression pattern was characteristic of a D category IV blood group variant.³⁵