There are two places in and on a domestic cat where a food allergy can show up (1) the skin and particularly the ears and (2) the gastrointestinal tract (GI).
I think it is important to recognise that a food allergy can manifest itself in the ears of a domestic cat. It is not a connection that a cat owner would immediately make. But some dermatologists, think that ear infections affect up to 50% of cats suffering from a food allergy. They become red and itchy. The real problem comes because domestic cats can very efficiently scratch their ears with the claws of their hind paws. They can do this vigorously. Domestic cats don’t know when to stop or they don’t realize that they’re doing damage to their skin when the irritation is very severe.
Importantly their feet may carry poop from the litter box which contains bacteria which can then be deposited onto the broken skin caused by scratching. This leads to a secondary bacterial infection which of course makes the problem twice as bad.
The symptoms of a food allergy on the skin may be in the form of rashes particularly around the face, head and ears. There might be a bald spot in front of the ears. The eyelids may swell up. There may be hair loss and oozing sores due to constant scratching and biting.
This is a common sense reference because it is obvious that a domestic cat will have digestive problems because of a food allergy. The visible symptoms will usually be diarrhoea or vomiting. The cat may develop inflammatory bowel disease (IBD) or lymphoma.
As an aside, it has come to my mind that people ask whether Ragdoll cats have a sensitive stomach. I did some research on this on the Internet. There are no scientific studies and therefore we have to rely upon anecdotal evidence. Ragdoll owners can provide good evidence. There is a Ragdoll forum where this is discussed. In my estimation about two thirds of the owners said that their cat had a sensitive stomach or the indications were that that is the case. However, we don’t know what caused these problems.
Perhaps the Ragdoll cat owners were providing the wrong cat food for their individual cats. Ragdoll cats are purebred cats but all cats have the potential for food intolerance. It is an individual cat thing. Some cats are unable to tolerate certain foods or specific brands of commercial cat food. The cat owner has to discover this by trial and error.
Therefore, it is impossible to say that Ragdoll cats have a propensity for food intolerance or food allergies without further evidence. A very good book that I have on genetic diseases in cats does not provide any information on Ragdoll cats and food intolerance.
Food allergy dermatitis
It is worth pointing out that the itching referred to above in the ears and over the head, neck and sometimes the back is due to food allergy dermatitis.
Sources: Myself, Dr. Hofve, Cat Owner’s Home Veterinary Handbook, ragdollcatguide.com, Medical, Genetic & Behavioral Aspects of Purebred Cats by Ross D Clark