Final answer:
The placenta is a crucial component during pregnancy, protecting the fetus by preventing the transfer of most potentially harmful substances and microorganisms from the mother, but allowing necessary exchanges of nutrients and waste. It prevents mingling of maternal and fetal blood, which is important for avoiding immune reactions such as Rh incompatibility. However, some pathogens and fetotoxic substances can still cross the barrier, and alcohol consumption by pregnant individuals is known to lead to fetal alcohol spectrum disorders (FASD).
Step-by-step explanation:
During pregnancy, the placenta serves as a critical interface between the mother and the fetus. It facilitates the transfer of oxygen, nutrients, and waste between the maternal and fetal blood without allowing their blood to commingle. The placenta also acts as a barrier to protect the fetus, preventing many potentially harmful substances from passing through. This includes microorganisms that could cause infections, protecting the fetus from diseases. However, certain pathogens such as the gram-positive bacterium Listeria monocytogenes can cross the blood-placental barrier, posing risks to the fetus. Moreover, the placenta is permeable to lipid-soluble fetotoxic substances like alcohol, nicotine, and some medications which can be harmful or even fatal to the developing embryo or fetus. Consequently, the consumption of these substances can lead to conditions such as fetal alcohol spectrum disorders (FASD) among others.
The placental barrier is designed to prevent maternal blood cells from entering the fetal circulation, avoiding the destruction of fetal cells that carry different antigens ("non-self" antigens). This separation is also crucial for preventing an Rh-negative mother from developing antibodies against an Rh-positive fetus, potentially causing hemolytic disease of the newborn. Although blood cells do not cross the placenta, the organ is well-adapted to allow the necessary exchange of substances through the chorionic villi. Antibodies produced by the mother for passive immunity do successfully cross the placenta, providing the infant with temporary protection after birth.