What is the main source of natural immunity for newborns?

The correct answer is maternal antibodies. Newborns receive natural immunity primarily through the antibodies passed from the mother during pregnancy and, in some cases, through breastfeeding after birth. These antibodies, specifically immunoglobulin G (IgG), can cross the placenta and provide the infant with passive immunity, helping protect them from infections and diseases during the early months of life when their own immune system is still developing.

While breast milk is beneficial and provides additional immune support through the presence of other antibodies and nutrients, it is the maternal antibodies that form the main source of early protection. Environmental exposure and genetic factors play smaller roles in immunity during infancy, but they do not provide the immediate, specific immunity that maternal antibodies offer to a newborn. This is why maternal antibodies are highlighted as the main source of natural immunity for newborns.