What causes your body to suddenly develop an allergic reaction to something later on in life that you weren’t allergic to before?

Question

i.e. how does ur body suddenly decide something is dangerous enough to warrant a reaction even tho it didnt cause it before

i might be using these tags wrong yeesh

In: 42

Answer 1

Around 12 years old I became allergic to bananas, melons (not watermelon, thank god), and pistachios. Every now and then I try them to see if they still cause a reaction and yep, still 20 years strong