The cells which attach to the mother's uterus continue to develop and be nourished in this secure place. But this is an amazing thing, because the quickly growing embryo is normally confronted by a serious danger—the mother's immune system.
The immune system regards every kind of foreign material entering the body as an enemy and attacks it. The embryo's genetic make-up is different from that of the mother and, for her body, it is a foreign organism. The moment the mother's defensive cells become aware of the presence of this foreign organism, they rush towards the uterus. If no special precautions were taken, the defensive cells would surely kill the embryo.
But there is no such occurrence under healthy conditions, because the embryo is taken from the beginning under special protection.
Before the embryo attaches itself to the wall of the uterus, trophoblast cells begin to form around the surface of the embryo, forming a kind of filter between the mother's blood vessels and the embryo. The mother's immune cells are unable to detect the trophoblastic tissues because they lack some proteins that most other cells carry and which help the immune cells to detect them. Thanks to this characteristic of trophoblast cells, the embryo is protected from assault by the maternal immune system. Moreover, some of the trophoblast cells assist in causing oxygen, nutrients and other necessary substances to reach the embryo.
Now, let's examine in detail the special structure of these cells.