Allergy is an immune response. Intolerance is a digestive system response. Symptoms may be similar but the are different things.
http://www.webmd.com/allergies/foods-allergy-intolerance
Interesting fact I learned back when I worked for a Vet. Cats often will only eat the shape of food they are used to. It’s the reason for all of the funny shaped cat foods, as opposed to dog foods which tend to all be the same shape (and dogs could generally care less, they will eat anything). Still, cats’ natural food is not potatoes or grains.
ANY inflammation is an immune system response. Even the swelling around a cut or bruise. If an inflammatory response involves the digestive system, then it’s also a digestive system response (and the full response often involves more than the immune system).
As for my kitty, he couldn’t care less about the shape of food — anything that’s dry cat food, he’ll eagerly dive into, from cheapo X-shaped Friskies to small pellet-shaped Hill’s Science Diet. Flavor makes no difference to him either. He didn’t hesitate for a second when I poured this new food into his bowl, even though he’s never in his life encountered a grain-free, potato-containing cat food. Small dry brown things rattling into bowl is his trigger to eat. If I poured brown aquarium gravel into his bowl, he’d probably eat that too. But offer him a bowl of human tunafish from a just-opened can, and he just gives you a perplexed look, like you’d set a brick down in front in front of him.
I’ve started to seriously suspect that he may have no sense of smell. A couple of weeks ago, in desperation, I started an intensive effort to get him to eat Gerber’s chicken baby food. Though he’d walk away from it if I stuck it under his nose, I found that if I forcibly pried his mouth open and placed a dab on his tongue, THEN he’d get interested in eating what was in the bowl (sometimes not until the 2nd or 3rd forced dab, though, and oddly never seeming to associate the food with the negative experience of the forced tasting). This went on for about 10 days, and now FINALLY, he actually eats the stuff when it’s just presented to him in a bowl. In fact yesterday he started begging for it. Bummer, because it was just a few days ago that I started the grain free food, and just a couple of days ago that I was able to confirm that it 100% solves his loose stools problem (and will almost certainly solve his weight loss problem too).
I still can’t interest him in Fancy Feast, which all my other cats love. But I haven’t tried the forced tasting method yet. I may need to do that. He was perfectly healthy for many years on dry food alone, but as cats get into senior years, it becomes more important to be able to get supplements and medications into them by mixing it into wet food. He’s 12 now, so it’s soon likely to be an issue. Gerber’s is a great base, but requires a lot of laborious supplementation to be nutritionally complete for a cat. I’ve used it as the main food for a super-senior cat (20+) and it worked wonders, but I don’t want to deal with the big row of supplements every day for many many years (which I certainly hope my 12 year old has ahead of him).