Blood group A antigen in human erythrocytic membranes and membrane fractions.

Erythrocytic membranes from blood group A individuals were assayed for A antigen using a quantitative hemagglutination inhibition technique. The membranes were then extracted for lipid and glycoprotein. Although some A antigen was usually found in the glycoprotein fraction, most of the activity was in the lipid fraction. The sum of A antigen activity in the lipid, glycoprotein, and membrane residue fractions only occasionally was equal to the A activity in the erythrocytic ghosts. However, when certain lipid preparations with little or no A antigen (enhancement factors) were added to the glycolipid fractions, the amount of A antigen demonstrated was usually greatly increased. Under these conditions, the sum of the fractions often was much greater than the A antigen demonstrated in erythrocytic membranes. This suggests that the organization or arrangement of A antigenic determinants in the red cell membrane may not always permit a stoichiometric reaction with anti-A molecules.