How does the immune process that causes the hemolytic disease of the newborn take place?

In a hemolytic disease of the newborn the mother has Rh- blood and this mother when generating her first Rh+ child makes contact, possibly during delivery, with Rh+ red blood cells of the child and her immune system triggers the primary immune response against the Rh factor. In a next gestation in which the fetus is Rh+ the mother will already have much more anti-Rh antibodies in her circulation; these antibodies cross the placental barrier and gain the fetal circulation causing fetal hemolysis (destruction of the red blood cells of the fetus).