Passive acquisition of ABO antigens from human serum by PBMCs. PBMCs isolated from O, A, or B donors were cultured in 10% human serum from individuals of blood group A or B. The cells were stained for blood group antigen expression using the H-specific lectin (Ulex), a blood group A-specific monoclonal antibody, or the B-selective lectin BSI-B4 (open curves) compared with negative controls (shaded curves), no lectin (H antigen), isotype antibody (anti-A), or the same cells cultured in O serum (B lectin). Both lectins were FITC-conjugated, and a secondary anti-mouse FITC was used for the anti-A staining.