Maternal antibodies against fetal blood group antigens A or B: lytic activity of IgG subclasses in monocyte‐driven cytotoxicity and correlation with ABO haemolytic disease of the newborn

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Abstract
IgG antibodies against blood group antigens A or B (anti-A/B) are able to sensitize erythrocytes for destruction in an antibody-dependent cell-mediated (ADCC) assay with monocytes as effector cells. The activity of maternal IgG anti-A/B in this test was compared with clinical signs of haemolytic disease of the newborn (HDN). When the ADCC was negative (less than 10% of the sensitized cells lysed), signs of increased red-cell destruction in the children were never observed. In three cases with a strongly positive ADCC (>45% lysis), the children were severely affected and needed more than one exchange transfusion. In the cases with >10% but less than 45% lysis in the ADCC, there was no clear correlation between the result of the ADCC and the degree of lysis in the newborn infants. In these cases, the degree of lysis of the red cells of the infant was shown to be strongly influenced by the number of A/B antigens per red cell. There was a direct correlation between the degree of lysis in the ADCC and the titre of IgG3 anti-A/B in the sera. There was comparable activity of maternal IgG anti-A/B in the ADCC test in the 32nd week of pregnancy and at the moment of delivery.