eli5: why do humans have allergies (hay fever, pollen, etc)

Question

eli5: why do humans have allergies (hay fever, pollen, etc)

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Answer 1

Allergies are hypersensitivity of the immune system to the substances. Typically, early exposure to the allergens helps avoid them, and common therapy includes exposing the body to very small doses of the allergens hoping for the body to get used to it without the overresponse.
As for why this happens, well, it’s a body disorder similar to other ones