Why do relatives 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?
Answers:    It could be that they only aren't around whatever they are allergic to, so it seems as if it comes out of nowhere.
other 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 liquid eyes, a runny nose and sneezing, now she can't even be near wattle lacking breaking out in the same symptoms, where as earlier she could smell wattle without any problems.
She says that she became allergic at that time, but is it that she suddenly developed the allergy? or is it that it be always there but not strong enough to be apparent until she came in contact with closely of it, which then triggered her allergy into high gear?
Their immune system becomes compromised. This is adjectives I have.


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