Final answer:
It is true that IgG antibodies can cross the placenta, providing passive immunity from the mother to the fetus. The placenta acts as a selective barrier, allowing these antibodies through while preventing most blood cells and larger plasma proteins from crossing, with the notable exception of Rh antibodies in cases of Rh incompatibility.
Step-by-step explanation:
It is true that some antibodies can cross the placenta to the developing embryo or fetus. Specifically, antibodies known as IgG are small enough to pass through the placental barrier, providing the fetus with passive immunity from the mother. This transfer of antibodies is crucial for protecting the infant during the early stages of life before its own immune system is fully developed.
However, the placenta also serves as a barrier to prevent the mixing of maternal and fetal blood, which helps to avoid potential immune reactions. While most types of cells and larger plasma proteins do not cross the placenta, Rh antibodies, which are involved in Rh factor incompatibility, can cross the placental barrier. If the mother is Rh-negative and the fetus is Rh-positive, these antibodies can lead to hemolytic disease of the newborn (HDN) in subsequent pregnancies if left untreated.