Well, allergies are almost always "something you tolerated most of your life (until then)" ... in my understanding you're not born with immune-intolerance to so

Well, allergies are almost always "something you tolerated most of your life (until then)" ... in my understanding you're not born with immune-intolerance to something (although you are most certainly born with a disposition or lack thereof), and not always is the manifestation of an allergy your system's first encounter with that specific protein - it is simply chance. E.g. the possibly most prevalent allergies, against pollen, manifest well after kindergarten, or even in teenagers. Until then the system did not care at all about pollen. The reason must be something entirely unrelated to pollen. And there are more and more cases of people developing pollen allergies well after age 50! (But: often you will find other allergies or "pre-symptoms" in kids, e.g. neurodermatitis or milk-protein allergies). In fact you always develop the allergy on exposure AFAIK - it just might be the first exposure. But the reason is most often completely unrelated to this specific substance/protein. There are even several known substances that are known to cause allergies to develop on exposure (with high probability). But what you become allergic to is pure chance, e.g. could be the nickel-plated jewelry on your skin which you never had a problem with until then.