Final answer:
The primary antibody isotype transferred from mother to newborn through passive immunity is IgG, which crosses the placental barrier and protects the infant for the first few months after birth.
Step-by-step explanation:
The main antibody isotype that a newborn receives from its mother through passive immunity is IgG. This antibody is transferred from the maternal circulation to the fetus via the placenta. IgG is essential because it can cross the placental barrier, thus providing passive immunity to the developing fetus during pregnancy and protecting the newborn for the first few months of its life. In addition to this prenatal transfer, IgA antibodies are also passed to the infant through breast milk, offering protection against pathogens in the digestive and respiratory tracts.