The Definition of Two I Antigen Components1

The I blood group antigen has two main components named IFetal (IF) and IDeveloped (Id). The IF component is present on all human red cells including those of icord and iadult persons, and also on Rhesus monkey red cells. The ID component develops slowly on the red cells before birth and to a greater extent in the 18 months after birth. Inhibition studies with human milk show that strongly‐inhibitable anti‐I sera are of the anti‐ID variety, but only a minority of such sera are inhibitable. Comparative studies on the red cells of 12 newborn infants suggest that the same sequence of I antigen development occurs in them all, but different infants vary in the amount of I development that they have achieved at the time of birth. I antigen variants affect mainly the ID component, and it is this part of the antigen that may be weakened on the red cells of people with leukemia.