Reply #44: that's essentially correct....

[mike_c]

The mother has specific, genetically determined molecular markers on all of her body cells that her lymphocytes use to identify her tissues as belonging inside her body. The baby also has similar genetically determined markers, but since the mother and the baby are not genetically identical, the fetus's markers would not be recogized by the mothers lymphocytes, so they would stimulate an immune response. BTW, the fetus and the mother do not exchange blood at all under normal circumstances.

on edit: shutting this "self recognition" system down is a strategy that numerous parasites use to avoind stimulating an immune response from their host.