Red cell antigen expression in different developmental programs. (A) Expression of different red cell antigens by flow cytometry in primitive, definitive, and FL-derived RBC. Negative control is shown in gray. (B) Representative pictures of gel card assay using primitive and definitive isogenic iRBCs. RBCs were incubated with IgG against specific red cell antigens or control IgG. A positive reaction (agglutination) is indicated by the retention of RBCs at the top of the gel, whereas a negative reaction results in RBCs pelleted at the bottom of the tube. Expected outcome based on iPSC genotype is indicated.