Final answer:
The passive immunity provided to newborns through the placenta is attributed to the IgG class of immunoglobulins, which is crucial for protecting both the developing fetus and the newborn in their early months.
Step-by-step explanation:
Passive immunity provided to newborns through the placenta belongs to the IgG class of immunoglobulins. The role of IgG is critical as it is the only antibody class that can cross the placental barrier from the mother's blood to the fetal blood supply, thereby providing passive immunity to the developing fetus during pregnancy. Not only does IgG protect the fetus from infection, but it also continues to protect the newborn for a few months after birth, as it is the main antibody of secondary immune responses in the blood. While IgA is transferred through breast milk and protects against digestive and respiratory pathogens, it is IgG that plays a pivotal role in fetal and early infant immunity through placental transfer.