On the retention of the fetus as a homograft.

The conceptus is tolerated in spite of the antigenic dissimilarities between mother and fetus. A new hypothesis to account for this phenomenon is put forward. It is suggested that the vessels within the trophoblastic villi--analogous to the vessels of malignant tumors--are permeable to humoral substances but impervious to cellular elements. This property allows for the exchange of nutrients and metabolites but, at the same time, precludes the passage of leukocytes. The block to diapedesis is believed to be due to the deposition of an occlusive layer of alpha 2-macroglobulin on vascular endothelium. The source of this protide is the fetal liver. In the absence of diapedesis there is no homograft reaction within the placenta. The "graft" is tolerated.