"There is a theory that the reason we have so many food allergies is that our houses are too clean. Allergies are based on the IgE system, which is primarily an antiparasite system. Parasites often have systems to trick the immune system, so the IgE system looks really hard to find something to attack. Apparently in some people if there are no parasites the IgE will still find something to attack and you get an allergy. It makes sense, because for about 400 million years our ancestors always had intestinal parasites and only in the last century have we gotten things clean enough to get rid of them."
Although it is true that many parts of the immune system that can be overactive in allergy sufferers are also what are active against parasites, we don't fully know exactly what the IgE system is designed for, so I wouldn't say that IgE is an "antiparasite system".
The "hygiene hypothesis" is the theory you are referring to and it actually does not specifically have anything to do with parasites, whether they are intestinal or not. The hygiene hypothesis was formulated because it has long been known that urban dwellers have a higher incidence of allergy as compared to rural dwellers (specifically, people who grew up on farms). It is believed that infections at a younger age or at least exposure to certain bacterial toxins does reduce the risk of developing allergies. The theory is that if the immune system is not given something to do at a young age while it is developing, it could develop an aversion to things that it doesn't really need to fight (i.e. dust, cat dander, pollens, foods, etc.).
Now, all of this is still just a theory, and there is conflicting data, for example, on what having pets in the home does to your risk of developing allergy as an infant, so I wouldn't go around giving your children infections or parasites in their cereal.
"We do have a hereditary issue with nuts - her grandfather - but with her other allergy - eggs - no heredity issues at all."
People do not inherit allergies to specific things (like nuts, eggs, or cats), but they do inherit the *risk* for allergies. In other words, the more relatives you have that have allergies at all, the more likely you will have allergies; however, what you are allergic to may not have anything to do with what your relatives are allergic to. It is the trait to *be* allergic that is inherited, not the actual things that other members of your family were allergic to per se.
One curious thing that I've noticed in my past practice, and something that was shown to be true in at least one published study I read, was that if your *spouse* is allergic, you have an increased chance of complaining of allergies *after* being married to them. There is no medical explanation for why that would be. I have my own theories.
