Fur baby allergies are weird AF.
When we met, Mrs. George adopted one my post-divorce cats - Chervil. We had to re-home her because of apartment restrictions.
We moved to a place with no restrictions and got two cats (Sadie and Oscar). Her asthma got so bad with these two cats that Benadryl did nothing. She was injecting sub q epinephrine (!) to stop the wheezing. We had to get rid of the cats.
When D3 had to re-home The Bee, she asked us. Shockingly, there we no reactions. The only thing I can think of is that The Bee was a long-hair cat. Somewhere, I read that long-hairs are less allergenic. After The Bee died, we got Maggie (another domestic long-hair) with the caveat that we could return if there was a problem. There wasn't.
Mrs. George attributes it to allergy shots she got. That may be the case.
There's no issue with Boris.
Go figure.