According to Hinde, 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 decipher it.8 This “baby spit backwash,” as she delightfully described it, contains signals, information about the baby’s immune system—including any infections it may be fighting. Everything scientists know about physiology indicates that this baby backwash is one of the ways that breast milk is able to adjust its immunological composition.
