[mathieuh]

It’s known that repeated exposure to allergens can cause allergic symptoms in people previously without them. For example https://en.wikipedia.org/wiki/Bird_fancier%27s_lung https://en.wikipedia.org/wiki/Farmer%27s_lung

I actually seemed to grow out of hay fever when I was in my early 20s. Perhaps coincidentally this is also around the time I developed an allergy to cannabis from overuse. Wonder if they’re related somehow.

[IAmBroom]

Poison ivy/oak sensitivity varies with lifetime exposure. On rare occasions, it is inverse: careful exposure can sometimes lead to resistance. More commonly: you get more sensitive over your lifetime, and some "once immune" people end up catastrophically changing due to incautious exposure.

It's complicated.

[bobthepanda]

it's a bit of a balance. based on this advice, there was a long time recommendation to avoid giving young children peanuts. however, this advice was rescinded after it turned out to increase peanut allergies.