By following the titer of passively acquired Rh antibodies in newborn infants, it is calculated that the half-life of the placenta-passing Rh antibody molecule is approximately 30 days. Evidence is offered that this estimate applies not only to placenta-passing Rh antibodies, but also to other placenta-passing antibodies, and not improbably to serum gamma globulin molecules in general.

Studies on the fate of free Rh antibodies in erythroblastotic babies show that these antibodies often are eliminated no more rapidly in Rh-positive babies than in Rh-negative babies. On the other hand, incompatible alpha and beta antibodies, as a rule, are quickly neutralized and eliminated from the body of newborn infants. These observations provide one explanation for the lower incidence of erythroblastosis due to A-B sensitization in comparison with cases resulting from Rh sensitization.

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