Final answer:
The most prevalent antibody that can cross the placenta is IgG, which is crucial for providing passive immunity to the fetus. It is the main antibody involved in both primary and secondary immune responses, and it is naturally transferred from the mother to the infant.
Step-by-step explanation:
The most prevalent antibody, capable of crossing the placenta, is IgG. This antibody monomer is the most abundant in serum, accounting for about 80% of the total serum antibodies. IgG not only penetrates efficiently into tissue spaces but also is notable for being the only class of antibodies that can cross the placental barrier, thereby providing passive immunity to the developing fetus during pregnancy. The versatility of IgG extends to various functions within the body's immunological defense against pathogens. It is a key player in late primary immune responses and the main antibody for secondary responses in the blood due to class switching. Naturally acquired passive immunity involves the mother passing antibodies to the fetus across the placenta, of which IgG is the central component. Through this process, newborns have a high level of antibodies at birth, affording them protection from common pathogens while their own immune systems mature. In addition, passive immunity extends beyond birth, as IgG can continue protecting the infant for several months.