Phagocytosis of maternal lymphocytes by the foetal trophoblast and the immunology of pregnancy

Immunologists have been striving to find an effective theory to explain the survival of the mammalian foetal allograft ever since the late 1950s. Embryologists had revealed that the blastocyst implants in the uterine decidua by a process involving cytolysis and phagocytosis. Brewer in 1937 had demonstrated a phagocytic property of the cytotrophoblast after implantation directed at all of the circulating blood cells. We advance the theory, supported in part by our recent studies using labelled leukocytes in pregnant mice, that the cytotrophoblast is phagocytic throughout most or all of pregnancy, and that after implantation a major source of nutrition of the embryo is phagocytosed blood cells. In particular we theorize that alloreactive lymphocytes are very likely to be phagocytosed. We also theorize that there are phagocytic foetal cells derived from the placenta in the foetus as a last line of defence against alloreactive cells from the mother. Some of the maternal lymphocytes seen in the foetus could have a surveillance function over abnormal cells arising from time to time in the foetus. Nonetheless runt disease will not occur in the embryo because of the elimination of alloreactive cells, a process which in theory can continue after the birth of the young.