IgG subclasses were determined in 138 A or B infants weighing over 2,500g, born to O mothers. Direct
antiglobulin test (DAT) was positive in 43 infants and negative in 95 with anti-A and/or anti-B antibodies detected by
heat elution test. In 59 out of 131 infants without ABO hemolytic disease (ABO-HDN), no IgG subclass was
detectable. In the 72 others, IgG1 was found in 29/72, IgG2 in 63/72, and IgG3 was not detected. In 7 infants with
ABO-HDN, DAT was positive in 4 and negative in 3. In conclusion, in DAT-positive infants without HDN, IgG1 or
IgG2 may be bound to erythrocytes, but the amount of IgG1 is too small to cause hemolysis. In DAT-positive
ABO-HDN the amount of IgGl is sufficient to cause hemolysis. In DAT-negative ABO-HDN, IgG3 is responsible for
hemolysis, even though undetectable by DAT.
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