If a mother has Rh-negative blood and her foetus has Rh-positive blood it will result in rhesus incompatibility and lead to erythroblastosis fetalis. What will happen if the reverse occurs, when a mother is Rh+ and the foetus is Rh-?
In a way, it's comparable to blood types in that regard. People with the blood group O can develop antibodies against type (for example) B blood. People with type B don't develop antibodies against type O blood, and can thus receive blood donations of type O.
Also, rhesus incompatibility during pregnancy is only a concern in second or subsequent pregnancies (or after miscarriages). The antibodies develop through the blood being "mixed" during the birth (or miscarriage) and are then an issue if the next fetus also has Rh+ blood, which is just assumed if the father is Rh+ or has an unknown blood type to be on the safe side, and can be addressed medically.
Injections of a medicine called Rh immune globulin can keep your body from making Rh antibodies. This medicine helps prevent the problems of Rh incompatibility. If you're Rh-negative, you'll need this medicine every time you have a baby with Rh-positive blood.
Also, as you can read in my answer to a similar question about blood groups and pregnancy How does a fetus retain a blood group different from its mother? the maternal and fetal blood usually only mix during birth due to the placental blood barrier. If you are wondering whether the fetus should have a reaction, the immediate functions necessary for that don't develop until more than six months after birth.