Allogeneic recognition and rejection reactions in the placenta.

One of the major questions in medicine is how human extraembryonic tissues escape rejection by maternal immune responses, because these tissues are allogeneic and should stimulate allogeneic recognition and rejection reactions. Evidence for maternal allogeneic recognition of extraembryonic tissues in chorionic villi, basal plate, and spiral arteries is presented. Extraembryonic membranes seem always to stimulate maternal allogeneic recognition and rejection reactions, and abnormal pregnancies appear to be associated with a simple quantitative increase of the same placental immunopathological lesions found in normal pregnancies.