They discuss a lot of bonding and ritual, but in the end:
“When a mother kisses her baby, she ‘samples’ those pathogens that are on the baby’s face. Those are ones that the baby is about to ingest. These samples are taken up by the mother’s secondary lymphoid organs like the tonsils, and memory B cells specific for those pathogens are re-stimulated. These B cells then migrate to the mother’s breasts where they produce just those antibodies that the baby needs.” says Lauren Sompayrac, author of How The Immune System Works.
Rock on, breastmilk!
Okay, that’s just awesome.