Why do people develop allergies out of nowhere?

when they lived their entire lives without allergy problems but then all of a sudden they do? how's that?

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Answer:
It could be that they just aren't around whatever they are allergic to, so it seems as if it comes out of nowhere.
There’s also the case of already being allergic to something but its so mild as to be unnoticeable, at least until something triggers a more severe reaction.
My mom is allergic to wattle(Acacia pycnantha) but she never used to be, one time she had to drive a van full of wattle for a few hours and along the course of the trip she developed red irritated watery eyes, a runny nose and sneezing, now she can't even be near wattle without breaking out in the same symptoms, where as before she could smell wattle without any problems.
She states that she became allergic at that time, but is it that she suddenly developed the allergy? or is it that it was always there but not strong enough to be noticeable until she came in contact with a lot of it, which then triggered her allergy into high gear?

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Answer:
Their immune system becomes compromised. This is all I’ve.