A 2-month-old female infant is brought to the pediatric clinic for a routine well-child visit. Her mother asks the pediatrician about the infant's immunity to infections. The mother's prenatal laboratory studies showed she was immune to rubella, with positive rubella IgG and negative IgM antibodies. The pediatrician explains that the infant received protective antibodies against rubella from her mother during pregnancy and is currently protected through passive immunity. The infant has not yet received the MMR vaccine. The pediatrician further explains that maternal IgG antibodies cross the placenta to provide this passive protection to the infant, whereas maternal IgM antibodies do not cross the placenta and are not detected in the infant's blood.
The ability of maternal IgG antibodies to cross the placenta and provide protection to the infant is primarily mediated by which of the following components of the antibody molecule?