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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. This "baby spit backwash," as she delightfully described it, contains signals, information about the baby's immune system-including any infections it might be fighting.
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Angela Garbes (Like a Mother: A Feminist Journey Through the Science and Culture of Pregnancy)