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There are 3 proteins that humans are allergic to in domestic cats. The main one is Fel d1, a protein secreted from the sebaceous glands and present on the skin and fur. It is also present in small amounts in cat saliva. The other two allergens are the proteins Fel d4 and Fel d7 these are both salivary proteins. Mostly you're exposed to these two via the cat's fur as they are deposited there during grooming, though being licked by a cat will also do it. Individual cats also express differing amounts of each of these proteins. So, if you are Fel d4 sensitive but a specific cat doesn't express any (or minimal amounts of) Fel d4 you'll not be allergic to that specific cat. Though you can be allergic to one, two or all three of the cat allergens.

Whether you would be allergic to big cats comes down to two things, do big cats also produce homologous proteins to the three domestic cat's proteins? And are the big cat versions of these proteins sufficiently similar to still invoke the immune response?

Well, it turns out that people have already tested some of this for Fel d1. And the answer is that if you are allergic to house cats your antibodies will bind to the big cat version of Fel d1 but that your antibodies don't bind as specifically/strongly to the big cats' versions of the protein. This gives us answers to our two questions; Yes, big cats produce a Fel d1 homologous protein that your anitbodies can bind. But, as the binding isn't as strong there's a good chance any allergic reaction will be weaker than your reaction to house cats.

https://pubmed.ncbi.nlm.nih.gov/1695231/

There seems to be a lot less information about Fel d4 and Fel d7. But I'd assume as all cats are pretty closely related the big cats likely have their own homologous versions of these proteins as well and somewhat similar results are also likely

I'd argue that the d1 homologous protein in big cats works differently, the reaction would be much more severe even fatal when coming in contact.

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> the reaction would be much more severe even fatal when coming in contact.

Why? As the paper shows, in Table 2, patient Fel D 1 specific IgE and IgG4 bind significantly less strongly (often by an order of magnitutde less) to the big cat proteins. I'll admit it's been a fair while since I studied much immunology but my understanding is less binding of Igs is associated with less immune system activation and less strong immunological responses.

(though of course a person could later go on to develop a specific allergy to a big cat fel d1 homologue)

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Out of curiosity, which one of these, if any, would correspond to pork-cat syndrome? (I have a pork intolerance or mild pork allergy but no apparent cat allergy or broader mammalian meat allergy like alpha-gal.)

After a quick google the answer is none of these. This is a rare condition where you have an allergy to cat or pork epithelium (typically it would be some protein in the skin or mucus membranes of the animal). And being exposed to the other causes a severe over reaction.

But if your pork allergy isn't to an epithelial protein then you wouldn't be at risk of being exposed to cat epithelia tissue

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4412402/

coming into contact with a big cat can be fatal, even ignoring the allergy aspect of it

"Autopsy notes: subject showed signs of nasal congestion, hives, disemboweling, and itchy eyes."

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Thank you for your excellent answers.