The majority of cases (65%) of hemolytic disease of the newborn have minor consequences and are caused by ABO blood-group incompatability between the mother and fetus. Type A or B fetuses carried by type O mothers is most likely to develop IgG antibody to the A or B blood-group antigens either through natural exposure or exposure to fetal blood-group A or B antigens in successive pregnancies. Usually the fetal anemia resulting from this incompatability is mild; the major clinical manifestation is a slight elevation of bilirubin, with jaundice. Depending on the severity of the anemia and jaundice, a blood-exchange transfusion may be required in these infants. In general, the reaction is mild, however, and exposure of the infant to low levels of UV light is enough to break down the bilirubin and avoid cerebral damage.
-Kuby Immunology, Sixth edition, pg. 391