Final answer:
A Rhesus D negative mother develops antibodies against Rh factor only after her first pregnancy. During the first pregnancy, the mother's immune system is exposed to the Rh antigen and starts producing anti-Rh antibodies. However, these antibodies do not generally harm the first Rh-positive baby. It is in subsequent pregnancies with Rh-positive babies that the mother's immune system generates larger quantities of anti-Rh antibodies, leading to the development of hemolytic disease of the newborn (HDN) if left untreated.
Step-by-step explanation:
During the first pregnancy, a Rhesus D negative mother's immune system is exposed to the Rh antigen present on the red blood cells of the Rh-positive fetus. This exposure triggers a primary immune response, leading to the production of anti-Rh antibodies. However, these antibodies do not usually cross the placenta and cause harm to the first Rh-positive baby.
It is in the subsequent pregnancies with Rh-positive babies that the mother's immune system is re-exposed to the Rh antigen. This exposure results in a stronger secondary immune response, leading to the production of larger quantities of anti-Rh antibodies. These antibodies can cross the placenta and attack the Rh-positive fetal blood, causing hemolytic disease of the newborn (HDN). HDN can result in anemia and other complications, potentially leading to fetal death if left untreated.