To the time you will give the cat to the shelter, you may look again for that allergen, as I said.
To revert the situation, persons that are allergic to cats, are allergic to all cats, not to a particular cat. That is because all cats generates the same allergens.
So I think is highly un-probable for the cat to be allergic to you, as a person.
It is also true that each individual has his/her own sensitivity to allergens. Also true children develop allergias that disappear in time.
In the books there are described the most probable allergens, most likely places or products. Books cannot be exhaustive.
But you may check with the shelter's vet before leaving the cat there (or perhaps by going there in a separate visit) if kitty suffers from allergia. For in your descriptions I see no proof, only deductions. Please do not tell to the vet the cat suffers from allergia! Describe the symptoms and present the cat to control. He/she (the vet) knows the profession, stating "it is an allergia" (making a diagnostic) is something that only a vet can do.
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Do not block yourself in your own deductions, sometimes we cheat ourselves.
Perhaps asking a good friend to inquiry you (like a detective may do :-)) about what you do during the day, both home and outside, to spot the possible sources of allergens. Sometimes we just cannot get out of a certain mind pattern. We need then someone else to search.
Oh trust me we have tried everything i can think of on the litterbox thats y i say she weird but i think all cats have a little weirdness in them thats y i love them i have a hard time saying no to a cat in need all my cats were strays we brought in except one she was in the adoption cage at petco for months so i had to bring her home they literally gave that one to me for free becouse nobody wanted her. I have five cats now because i cant say no but i reached my limit on how many i can help. Its sad i wish i could win the lottery and open my own ni kill shelter
Yes, every cat has their quirks. That's why we love them, and they each have their own place in our hearts
Part of adopting and rescuing cats is accepting our limits. We have to accept that we can only do so much.
That's why rescues are forced to limit the number of cats they accept.