The placenta has been considered an anatomical barrier which prevents maternal rejection of the fetal allograft. This view is no longer tenable since, in one particular inbred strain of mouse, maternal blood cells have been identified by means of radioisotope labels in newborn mice. Both maternal red cells and lymphocytes labeled, respectively, with radiochromium and tritiated thymidine have been detected in newborn mice. This phenomenon appears independent of the trauma of normal delivery and, what is more, occurs before term.

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