The developmental patterns of ABO saline isoagglutinins were determined in 272 children between birth and 16 years of age. The frequency of saline isoagglutinins was significantly higher in group O‐O maternal‐cord pairs compared to group A‐O paired sera. The anti‐A and anti‐B titers increased gradually from 3 to 12 months: 30 per cent of group O and B children attained adult median titers of isoagglutinin, whereas only 4 per cent of group A children acquired adult anti‐B titers. This slow development of anti‐B in group A children persisted through the second year. Maternal isoagglutinins may exert a suppressive influence on the development of specific isoagglutinins in the first year of life.

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