Let’s be honest: immunology is hard.

Sometimes I wonder why Biology decided to be so cruel to med students and scientists in general, making this part of the “human machine”, immunology, so complex. Truth is, we have to deal with it. Maybe one day all those pieces of information, disguised under the vest of acronyms and sophisticated mechanisms, will make sense to us.

The good news is that we still know very little of what is actually going on with our immune system (and with the immune system of most complex living beings). This should make us feel more intelligent (or less dumb, you pick your side). 

On the other hand, the unfortunate reality is that what we know about our immune system is complex enough to discourage further exploration.
Having said that, it still is undeniably beautiful. 

Ladies and gentlemen, take a look at one of the easiest-looking “selfies” of both our innate and adaptive immune system

👇

Well, this is a super-simplified snapshot which does not include a few thousands other funny, little names. But I suppose it will be sufficient for our topic today: 

Why do mothers kiss their babies?

Humans do it for a variety of reasons, but primarily because they love their children. But what about animals? They do just the same. As soon as the little bamby lands on planet Earth, there its mom will be ready to kiss (or better say, “lick”) that little face, still soaked in amniotic fluid. Why do they do that? Is it love then too?
Sure enough, a mother’s love is the only, real universal language there is. But biologically, and more specifically “immunologically”, the reason is something that will leave you speechless. Sit back and listen to this incredible story.

You see that picture up there? Do not look at it in its entirety. B lymphocytes comprise a distinct population of immune cells. These guys do a couple of interesting things, but above all they can differentiate into a specialised type of immune cells called “plasma cells”. Plasma cells are “antibody factories”. They are part of the adaptive immunity. We have four different classes of antibodies: IgM, IgA, IgG, and IgE.

Plasma cells do not produce antibodies randomly. If that was the case, we would be filled with antibodies which would serve no purpose. Instead, they “mount” these antibodies depending on which microorganism is trying to attack us, thus making the immune response microbe-specific.
In our intestines the most abundant antibody is IgA, which protects us against the constant and relentless attacks of microbes coming from the digestive tract. We are talking about a constant fight, with neither breaks nor truces. In fact, immunology could be seen as the science of how our body fights against threats of all kinds.

Newbors immunity is different

A newborn’s immunity is pretty weak. They only start producing their own defenses after a few months from birth. Before then, the mother’s IgG (which cross the blood-brain barrier) protect the baby for as much as they can. But the unique quality of IgA is that they are the only type of antibody that can be found in mothers’ breast milk. And guess what’s the main type of infections most babies (if not all) are affected by? GIT infections, causing classical symptoms of nausea and diarrhea.

The system works perfectly (immunology, what a gem!). The mother’s B-cells migrate to her breasts, start pumping IgA into her milk, the baby feeds on it, so ingesting large amounts of IgA ready to protect him from infections.
How beautiful is that?
But what about kissing? What has that anything to do with all this IgA transfer?
Hang on in there, we will get to that soon.

As we said, antibodies are built to defend against specific pathogens that individuals encounter throughout life. And that does not make sense in the case of mother-baby immunity transfer, because the baby does not need the type of IgA that the mother has built to fight against microbes that she got in contact with, not her baby.

And that’s why the kissing.
By kissing the baby, mothers take in pathoges from his/her skin, so stimulating her own immune system to produce IgA specific antibodies built for the little one! These IgA will then be transferred to the baby, protecting him (and the whole family!) from sleepless nights of unstoppable diarrhea.
And if the science behind kissing was a little too complicated, it does not really matter. Take my words and thank your mother: she literally had your back!