Final answer:
IgG is the antibody class capable of crossing the placenta, providing vital passive immunity to the developing fetus and the newborn for a few months post-birth.
Step-by-step explanation:
The class of antibody that is able to cross the placenta from the mother to the developing fetus is IgG. IgG is the only antibody class that can traverse the placental barrier, granting passive immunity to the fetus.
This transfer imparts the infant with immune protection, which can last up to six months post-birth.
This antibody is also the most abundant in human blood, making up about 80% of total serum antibody. It takes part in clearing pathogens from the blood, can activate complement proteins, and fights extracellular pathogens in tissue spaces.
In summary, IgG provides essential protection to the developing fetus from various diseases by transferring the mother's immunological memory of certain pathogens. Moreover, after birth, the infant receives additional protection through IgA antibodies present in breast milk which bolsters mucosal immunity against digestive and respiratory pathogens.