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The placenta plays a crucial role in protecting the fetus, primarily by acting as an immune barrier. While it does not entirely prevent all maternal infections, it selectively allows certain antibodies from the mother to cross into the fetal circulation. This selective transfer helps provide the fetus with passive immunity, particularly during the later stages of pregnancy, as maternal antibodies are transferred to provide some immune protection just before birth.

The placenta's role in immune modulation is significant; it helps shelter the fetus from the mother's immune system, which might otherwise recognize the fetus as foreign due to its different genetic makeup. This immune shielding is vital for a successful pregnancy and helps protect the developing fetus during the early stages before its own immune system is fully developed.

Context regarding other choices highlights that the placenta does not completely prevent maternal infections, as some pathogens can still cross the placental barrier. Additionally, while it does not increase the maternal heart rate, the placenta is involved in different physiological adaptations during pregnancy that can affect maternal cardiovascular status. Lastly, the placenta does not release insulin for fetal metabolism; instead, the fetus independently manages its metabolism, receiving nutrients and glucose primarily through the maternal blood supply.