The blood circulation of a two to three month old breast-fed baby will contain maternal ____________.

Maternal IgG is transferred across the placenta from the mother to the fetus by a mechanism involving the specific Fc gamma receptor FcRn IgG is the only immunoglobulin class to cross the placenta. Immunoglobulins in the milk (mostly secretory IgA) will provide enteric protection for the baby, but are not absorbed. Thus the maternal immunoglobulin in the circulation of a 2-month-old-baby will mostly be placentally-transferred IgG, which will provide 'cover' for the first few months of life.