Detection of the H and I blood group antigens in normal plasma. A comparison with A and i antigens.

The levels of A, H, I and i antigens were measured in the plasma of 185 normal subjects by the agglutination-inhibition method. The presence of H in the plasma was only detectable with immune anti-H. The level of H in the plasma was directly correlated with the amount on the red cells, and was affected by the donor secretor status. The plasma of group O secretors contained more H than the plasma of donors of other phenotypes. On the other hand, I and i plasma antigens were not related to the other systems studied. Unlike the Ii antigens on the red cell membrane, there was no relationship between the levels of I and i in plasma.