The distribution of the A-B-O blood groups among four hundred fifty-seven sensitized Rh negative mothers was compared with the distribution among eight hundred twenty-three nonsensitized Rh negative mothers. As expected, the frequencies of groups O and B were lower while that of group A was higher in the series of sensitized women. The difference in frequency of group A was 5 times its probable error and therefore statistically significant. Surprisingly, however, the difference proved to be due entirely to subgroup A1, while the frequency of subgroup A2 was the same in both series.

In a random series of four thousand nine hundred seventy-four Caucasoid individuals in New York City, the frequency of type rh’ was found to be 0.95 per cent. Statistical analysis of the Rh types of four hundred fifty-seven sensitized Rh negative mothers indicates, on the other hand, that the frequency of type rh’ is actually 0.41 per cent. The discrepancy is explained by postulating that in the control series, about half of the supposed type rh’ individuals were really type Rh1, since no special test had been made for Rh0 variants. In the series of sensitized Rh negative mothers, individuals of type Rh1 are automatically screened out since when factor Rh0 is present sensitization to the Rh0 factor is hardly likely to occur.

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