As if we need another god reason to breastfeed newborns, but here you go anyway... excerpted from "Antibodies in Breastmilk Prime the Baby's Gut to Handle Mom's Invading Microbes."
“… the breast milk is also loaded with microbes, which means a brand-new baby’s gut is subject to a full-on invasion of tiny, foreign animals from the moment he or she starts feeding. The infant’s immune system, rather than attack these foreign organisms like an immune system is designed to do, instead sits back and lets them invade. And now we know a bit more about why: In a study published Thursday in the journal Cell, researchers from the University of California Berkeley found that in addition to all those microbes, mother’s milk is loaded up with antibodies to the very same microbes it contains. In other words, the milk contains both the beneficial microbes and what the body needs to be able to accept them.”