Final answer:
False negative Rh testing results can occur in babies with severe HDFN (hemolytic disease of the newborn) due to anti-D because of a weak immune response during the first pregnancy, which changes in subsequent pregnancies.
Step-by-step explanation:
False negative Rh testing results can occur in babies with severe HDFN (hemolytic disease of the newborn) due to anti-D because the first exposure of the mother to the Rh factor antigens may not produce enough antibodies to trigger a significant immune response during the first pregnancy. Therefore, the Rh+ baby is not affected by the mother's antibodies. However, in subsequent pregnancies with an Rh+ fetus, the mother's immune system produces a stronger secondary antibody response, resulting in larger quantities of anti-Rh factor IgG antibodies. These antibodies can then cross the placenta from the mother to the fetus and cause HDN, a potentially life-threatening condition for the baby.