Rh factor

On monday in class we some how got onto the topic of the Rh factor and how if a woman is Rh negative and she gets pregnant there is a possibility that there could be problems. Although the blood doesn't normally mix between mother and fetus, it can occasionally. If it does and the baby is Rh + the Rh - mother will produce antibodies to the baby's Rh factor. The antibodies can cross the placenta barrier and attack the baby's blood causing anemia.
In order to prevent this, Dr.'s reccomend RH - mothers to get an injection of Rh immunoglobulin (RhIg), a blood product that can prevent sensitization of an Rh-negative mother at 28 weeks and then again right after delivery. The injections don't change the overall immunity of the mother, but rather only affect her immunity to the Rh factor.
here is an article that explains it better:
http://www.americanpregnancy.org/pregnancycomplications/rhfactor.html