eli5: Why does our immune system suddenly react safe substances making us allergic?

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So let’s say you’ve been eating nuts your whole childhood. You have had nuts added to different types of food. Your body has been exposed to nuts for quite a while.

What changes, for you to suddenly wake up one day and be allergic to nuts?

In: Biology
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As far as I know, it’s a current field of research. The entire process of how allergies suddenly start out of nowhere in someone’s life is not well understood yet. We do understand how an allergen eventually triggers an allergy, but why the body suddenly starts considering a certain molecule as an allergene, I don’t think we have an answer for that. But I might be outdated in this knowledge, that was the consensus some years ago when I was still in college.

Nobody knows!

There’s some correlation between being raised “too clean” and developing allergies. Playing outside and being sent to kindergarten reduces the odds of developing allergies.

but that doesn’t really explain them appearing later in life, or some of them disappearing with puberty.

So the honest answer to your question is, No idea, its being looked into.

This definitely doesn’t cover all allergies that develop later in life, but one mechanism would be when a previously tolerated antigen (like a protein from a peanut) is modified in some way (e.g. by some sort of abnormal chemical reaction in your body) such that your body no longer recognises it and views it as foreign. This could happen for multiple reasons:
1. You’re currently taking medication which either binds to the peanut allergen in a process known as haptenization or chemically modified it otherwise.
2. For allergens on the skin, they can be modified by UV light.
3. A bunch of other mechanisms I don’t know off the top of my head

It also has to do with other factors, such as the integrity of your body’s barriers to infection like your skin and the overall state of your immune system at the time of exposure to the allergen. If you have breaches in your skin, a concurrent infection (a cold or something), and the allergen is introduced through that breach (maybe a papercut on your hand) then your body may misinterpret the allergen as arriving in the context that a pathogen usually would.