The blood group I and i antigens of amniotic fluid. I. Association of I and i antigens with blood group A, B and H antigens.

Human amniotic fluid has been shown to contain blood group i as well as I antigens. Crude extracts of amniotic fluids at 16-23 weeks of gestation were in general more active than those obtained at term. A pool of amniotic fluids which had A, B, H as well as I and i activity was fractionated with an insolubilized anti-I (Group 3 type) immunoadsorbent column. There was evidence for the occurrence of I and i determinants on macromolecules carrying A, B and H determinants, for the fraction specifically retained by the column was enriched by 50-60-fold in I and i activity and by at least 10-fold in A, B and H activity. In the fraction not retained by the column there remained the bulk of the A, B, H activity in addition to I activity of Group I type which is known to be distinct from the Group 3 determinant. With the aid of specific immunochemical fractionation procedures, human amniotic fluid should prove useful in structural studies of the several I and i determinants and of their relationship to other blood group determinants.