Differing from humans, IgG from breast milk in many animal species (rodents, bovines, cats, ferrets, etc.) are transported across the intestinal epithelium into the neonatal circulation. This transport is located at the duodenal and jejunal level where enterocytes express a surface membrane receptor able to bind Fc of IgG and to facilitate transcytosis of these immunoglobulins. Fcgamma-R, which is very similar to the placenta receptor responsible for active transplacental transfer of IgG in humans, binds IgG but not other isotypes. Maternal milk antibodies represent an important part of circulating IgG in these animals, as they are involved in the negative feedback of endogenous IgG synthesis. This phenomenon stops abruptly as soon as weaning takes place. Neonatal calves that have a defect in such transfer of maternal immunoglobulins are at high risk of systemic infectious diseases. In humans, in whom gut closure occurs precociously, breast milk antibodies do not enter neonatal/infant circulation. A large part of immunoglobulins excreted in milk are IgA that protect mainly against enteric infections. The specificity of maternal milk IgA is driven by an entero-mammary cell circulation. Human milk also contains anti-idiotypic antibodies capable of enhancing infant antibody response. Maternal milk antibodies coat infant mucosal surfaces and some have a clear protective role. This has been studied extensively in infectious disease models such as rotavirus, E. coli, poliovirus, and retroviruses. In the rotavirus model, antirotaviral IgA can be detected in stools of breast-fed but not bottle-fed neonates. In a large cohort of lactating women infected with HIV-1 in Rwanda, anti-HIV milk antibodies of the IgG isotype were more frequently detected followed by secretory IgM. Surprisingly, anti-HIV-1 SIgA were less frequently found. The presence of milk SIgA at 15 days as well as the persistence of a SIgM response during the whole lactation period was associated with lower risk of HIV transmission from the mother to the infant. Recently, HIV-1 antibodies from maternal milk have been shown to block transcytosis in vitro in a monolayer enterocyte model. Among these antibodies, those directed against the ELDKWA epitope had higher neutralising activity than serum antibodies. In humans, milk excreted antibodies play a major role in protecting infants from infection by pathogens having a mucosal portal of entry.
Source: Vaccine. 2003 Jul 28;21(24):3374-6. http://www.ncbi.nlm.nih.gov/pubmed/12850343