"when a baby suckles at its mother's breast, a vacuum is created. Within that vacuum, the infant's saliva is sucked back into the mother's nipple, where receptors in her mammary gland read its signals. This 'baby spit backwash,' as she delightfully describes it, contains information about the baby's immune status. Everything scientists know about physiology indicates that baby spit backwash is one of the ways that breast milk adjusts its immunological composition. If the mammary gland receptors detect the presence of pathogens, they compel the mother's body to produce antibodies to fight it, and those antibodies travel through breast milk back into the baby's body, where they target the infection."

I found this fascinating!