Direct antiglobulin reaction in ABO-haemolytic disease of the newborn

The minimum number of IgG anti-A (or anti-B) molecules detectable on A or B red cells by the antiglobulin reaction was found to be the same-that is, about 150 molecules per red cell-with newborn as with adult cells. Furthermore, the ratio of anti-IgG bound to IgG anti-A (or anti-B) molecules was the same whether the anti-A (or anti-B) molecules were present on newborn or on adult cells and was similar to that found for anti-IgG bound to IgG anti-Rh.In 15 infants (11 group A, 4 group B) with haemolytic disease of the newborn due to ABO-incompatibility the amount of anti-A or anti-B on the red cells ranged from 0.25 to 3.5 mug antibody per ml red cells, corresponding to 90-1,320 antibody molecules per cell; only five infants had more than 0.55 mug antibody per ml of red cells. These amounts are far smaller than those found in most moderate or severe cases of Rh-haemolytic disease.It is concluded that the weak direct antiglobulin reactions observed in ABO-haemolytic disease are due simply to the fact that the number of anti-A (or anti-B) molecules on the infant's red cells is at the lower limit of sensitivity of the test. Since ABO-haemolytic disease can be quite a severe process it seems probable that IgG anti-A and anti-B molecules are more effective than anti-Rh molecules in bringing about red cell destruction.