A process where a mother with Rh-negative blood is pregnant with a baby that has Rh-positive blood and the baby's blood enters the mother's blood circulation. The Rh-positive blood from the baby makes the mother's body create antibodies. The antibodies will attack any Rh-positive blood cells. This will not cause a problem for the mother. However, the antibodies can pass to the developing baby and destroy some of the baby's blood cells.