Final answer:
IgG is the class of antibody that can cross the placenta, contributing to the passive immunity of the fetus.
Step-by-step explanation:
The major class of antibody that crosses the placenta is IgG. This antibody is the most abundant in serum and plays a crucial role in the immune defense against pathogens. It's a monomeric antibody that can penetrate tissue spaces and provide passive immunity to the developing fetus during pregnancy by crossing from the mother's blood to the fetal blood supply.
IgG is crucial during both primary and secondary immune responses, and it activates complement proteins, contributing to its antibacterial activities. Notably, IgG is the only class of antibody that provides natural passive immunity to the infant, effective up to six months after birth.