Can someone explain a Rh factor or isoimmunization to me?

Antibodies are key! If you have a negative blood type and you are pregnant with a baby that has a positive blood type, your body makes antibodies. In subsequent pregnancies, if the baby has a positive blood type, the antibodies attack the baby and can cause miscarriage or still birth. A shot during pregnancy and at the end of any pregnancy (including miscarriage or abortion) can prevent the antibodies from forming.