IgG Subclasses of Anti‐A and Anti‐B Antibodies Bound to the Cord Red Cells in ABO Incompatible Pregnancies

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Abstract
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 IgG1 is sufficient to cause hemolysis. In DAT‐negative ABO‐HDN, IgG3 is responsible for hemolysis, even though undetectable by DAT.