Which class of antibodies primarily crosses the placenta?

The class of antibodies that primarily crosses the placenta is IgG. This is significant because IgG provides passive immunity to the fetus during pregnancy. It is the only antibody class capable of efficiently crossing the placental barrier, which allows maternal antibodies to be transferred to the fetus. This transfer helps protect the newborn from infections during the early months of life until the infant's own immune system becomes fully functional.

IgA is predominantly found in mucosal areas and secretions, such as saliva, tears, and breast milk, and does not cross the placenta effectively. IgM is the first antibody produced in response to an infection and is too large to cross the placental barrier. IgE is involved primarily in allergic responses and defense against parasitic infections and also does not cross the placenta. Thus, among the antibody classes, IgG plays a critical role in fetal immunity, making it the correct answer to the question.