Final answer:
IgA is the antibody class present in breast milk, which crosses epithelial cells and provides passive immunity to infants by protecting against pathogens in mucous membranes and secretions.
Step-by-step explanation:
The antibody class that is able to cross epithelial cells and is therefore present in breast milk is IgA. Specifically, the dimeric form of IgA is secreted by exocrine glands, and it plays a crucial role in protecting body surfaces by being present in mucous membranes and various secretions including breast milk. This transfer of IgA to the infant through breast milk provides the infant with passive immunity against digestive and respiratory pathogens.
Passive immunity is the transfer of active humoral immunity in the form of ready-made antibodies, from one individual to another. IgA in breast milk is a perfect example of natural passive immunity, where a mother passes vital antibodies to the infant, enhancing the infant's immune defense against infections.