Source: Allergy and the gastrointestinal system
Here is an article describing how complex our immune system is and pointing out that 80 to 90% of various fractions of it are present in the gut.
Even for a fetus that does not have a fully working digestive system because it’s not eating anything there is a huge immunological system developing in its gut. The fetus swallows amniotic fluid and primes its gut with systems involved in immunity and allergies.
the fetus has an immunological system orientated towards the T helper type 2 (Th2) responses, which protect it from ‘graft-versus-host’-type reactions, and that only following birth can the Th2 phenotype shift to a Th1 phenotype which provides protection from allergic reactions. This is modulated by microbial agents, and particularly by being given lipopolysaccharides (LPS) typical of Gram-negative bacteria. By contrast, poor exposure to such microbes and strict hygiene will orientate towards persistence of the Th2 phenotype, with facilitation of the IgE-mediated allergic response [4–6].
Graft-versus-host disease is when grafted immunologically active tissue attacks the host body and causes damage. For a fetus the host body is itself (the fetus) and the graph would be the mother’s immune system. The fetus would certainly like to avoid having the mother’s immune system kill it 😏. It is one of the mind-numbingly huge number of systems expressed and regulated by our genes that have to be constantly updated and tailored to our evolutionary pathway. (why-the-long-pause)
And reading this paper, I think it lends support to my original notion here that a mother can transfer peanut allergy _protection_ to her fetus. One of the key ideas here is that we need to be exposed to Gram negative bacteria, LPS, and allergens. It is entirely possible that our houses can be too clean. And this may not be the only route to allergen protection I still think antibodies in mothers milk are important but it could be that it is just through her blood and the amniotic fluid.
Biology is so Cool. ☺