The majority of infants under 6 months of age are apparently immune to measles, scarlet fever, diphtheria and poliomyelitis. This immunity is usually explained as the result of transmission of antibodies through the placenta, followed by postnatal transmission in colostrum and milk. In some cases, however, this simple explanation seems inadequate; infants, for example, appear immune to scarlet fever and diphtheria, whereas their mothers are demonstrably susceptible to these diseases. Again, umbilical blood from some new-born infants will neutralize poliomyelitis virus in spite of the fact that the blood of their mothers is without demonstrable virucidal properties. To account for such paradoxes, certain immunologists have postulated the existence of a special mechanism in infants, a "tissue immunity" associated with the rapid multiplication of fetal and infantile cells. If this fetal mechanism is a reality, the placenta presumably might be the source of antibodies useful in treatment.
Applying this conception, McKhann
PLACENTAL ANTIBODIES. JAMA. 1933;101(1):37–38. doi:10.1001/jama.1933.02740260039016
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