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.