If your red blood cells contain a substance known as rhesus factor, you are rhesus, or Rh, positive. If a woman who is Rh negative conceives a baby by a man who is Rh positive and the baby, in turn, is Rh positive, the woman may become sensitized to the Rh positive blood at the time of delivery. If a subsequent baby is also Rh positive, the mother’s body recognizes the baby’s “foreign” blood and produces antibodies that attack the baby.
Rh positive blood from the baby enters the mother’s bloodstream.
After first baby
Antibodies develop in the mother to combat Rh positive blood and afterward remain.
Antibodies enter subsequent Rh + baby’s blood, causing rhesus hemolytic disease.