I think it depends on the type of allergy and allergen which sets it off.
I was brought up around cats and some set me off more than others, moreso ones I hadn't been around a lot and would suffer more when I came back to visit than when I'd lived there. I also have my own cats and have done for years.
I have a diagnosed cat hair allergy which came up when I saw an immunologist to rule out a latex allergy for an operation. It came up one of the highest (assume maybe with the exposure). They diagnosed me with oral allergy syndrome as I have a number of them.
I've found that it's more if hair gets anywhere in my nose or mouth I suffer, like when I'm trying to get cat hair up from the carpet.
Same with horses (although the allergy is much more severe to them), it's if I touch them and get the hair anywhere near my face, especially my eyes. That one is probably the most extreme, eyes, breathing etc...
Similarly I have a nut allergy diagnosed same time but it's more the raw outside of them that seems to set me off (like cracking and eating out of the shell) and I'm ok with more processed ones like in chocolate.
Again, I came up allergic to apples. Some make my mouth go funny when I eat them, some I'm ok with.
The immunology nurse said an allergy which affects breathing can be the kind which can get quite severe quite quickly so she should be careful, especially if asthmatic.