Iso‐ and Heteroagglutinins in Human Fetal and Neonatal Sera

Maternal AB and sheep cell haemagglutinins were found in the human fetus after the 13th to 14th week of gestation. In fetuses aged 18 weeks or more they were observed as frequently as in full‐term newborns, 56.6% of which had isoagglutinins and 57.6% heteroagglutinins. The occurrence of maternal heteroagglutinins in neonatal sera was not dependent on the ABO group of the mother as it is with isoagglutinins. Heteroagglutinins of the IgG class were found equally in mothers of different ABO groups. Isoagglutinins against erythrocytes of the mother's group were detected in 8 out of 44 neonatal sera, but in none of 10 sera from fetuses aged 14 to 19 weeks. IgM concentrations in neonatal sera with and without isoagglutinins of fetal origin revealed that IgM synthesized by the healthy fetus consists mainly of antibodies other than isoagglutinins.