WHY MOTHERS KISS THEIR BABIES

Have you ever wondered why mothers kiss their babies? It’s something they all do, you know. Most of the barnyard animals also kiss their babies, although in that case we call it licking. I’m going to tell you why they do it.

The immune system of a newborn human is not very well developed. In fact, production of IgG antibodies
doesn’t begin until a few months after birth. Fortunately, IgG antibodies from the mother’s blood can cross the
placenta into the fetus’s blood, so newborn has this “passive immunity” from mother to help tide him over.
The newborn can also receive another type of passive immunity: IgA antibodies from mother’s milk. During
lactation, plasma B cells migrate to a mother’s breasts and produce IgA antibodies that are secreted into the milk. This works great, because most pathogens that a baby encounters enter through his mouth or nose, travel to
his intestines, and cause diarrhea. By drinking mother’s milk that is rich in IgA antibodies, the baby’s digestive
tract is coated with antibodies that can intercept these pathogens.

When you think about it, however, a mother has been exposed to many different pathogens during her life, and
the antibodies she makes to most of these will not be of any use to the infant. For example, it is likely that the
mother has antibodies that recognize the Epstein–Barr virus that causes mononucleosis, but her child probably
won’t be exposed to this virus until he is a teenager. So wouldn’t it be great if a mother could somehow provide
antibodies that recognize the particular pathogens that her baby is encountering – and not provide antibodies that the baby has no use for? Well, that’s exactly what happens.

When a mother kisses her baby, she “samples” those pathogens that are on the baby’s face – the ones the baby
is about to ingest. These samples are taken up by the mother’s secondary lymphoid organs (e.g., her tonsils),
and memory B cells specific for those pathogens are reactivated. These B cells then traffic to the mother’s breasts
where they produce a ton of antibodies – the very anti-bodies the baby needs for protection!