Direct antiglobulin test (DAT) and heat elution test were examined on the cord red cells of 257 A or B infants weighing over 2, 500g born to O mothers. DAT was positive in 43 infants (group I), while was negative in 95 infants which were detected anti-A and/or anti-B antibodies by elution test (group II), and remaining 119 infants were not detected any antibody in their eluates. IgG subclasses were determined in 138 infants of group I and group II. In 59 out of 131 infants without ABO hemolytic disease (ABO-HDN), all IgG subclasses were undetectable in their eluates. In the eluates of 72 others, IgG1 was detected in 29/72, IgG2 in 63/72, and IgG3 was not detected in any infant. In 7 infants with ABO-HDN, DAT was positive in 4 cases and negative in 3. In conclusion, in DAT positive infants without HDN the only IgG subclasses bound to erythrocytes are IgG2 or IgG2 with a small amount of IgG1 which cannot cause hemolysis, and in DAT-positive ABO-HDN the amount of IgG1 is enough to cause hemolysis. In DAT-negative ABO-HDN, IgG3 is responsible for hemolysis even though in amounts undetectable by DAT.
