The way I look at it, every dog is allergic to something. How many people can say that they tried a dozen dog foods, and over a period of six months on each of them their dog was fine on every one? Every dog will have a food that just doesn't "sit right." It may be the ingredient, the chemicals put into the ingredient, where the ingredient came from, how the ingredients are put together, etc.
Why else do so many people change foods for? "My dog had large stools on this food," "my dog wasn't shiny on this one," "my dog smelled bad on this one," "my dog was gassy on this food," "my dog gained weight on this food," "my dog's's glands were always full when fed this food," "my dog's hair felt greasy on this one." All symptoms of allergies.
How many people can truly say that they have fed the same dog food to all of their dogs since they had dogs and every one of their dogs has been "perfect" on it? And since you haven't switched to something else, how can you really compare to truly KNOW that your dog is perfect on it? And if your dog is perfect on it, how do you know it wouldn't be on something else? See where I am getting? I have known so many people who thought their dogs did not have any allergies until they were forced to switch foods, and suddenly their dog was farting all of the time. I've known many people who thought their dog didn't have allergies until they switched foods and suddenly their dog was more vibrant, had a more gloosy coat, higher energy, and leaner muscle. Until you start looking at things from a more natural or holistic view, people often don't see that allergies are more than what they seem. "Intolerances" can be seen in many different ways, even behavioural ones.

And if you don't think your dog has an allergy to ANYthing, you are wrong. You may think the same about yourself, and you are wrong. Everyone's body is intolerant of something and we show it in ways that we don't believe are symptoms of an allergy. Especially in ouir dogs, we recognise it as a "dog thing" but rarely see it as an allergy. If you go to a naturopath, he will test you and tell you what your body does not react well to. My friend recently went in and found out she is allergic to pineapple, citrus fruits, tomatoes, and wheat -- all things she ate on a regular basis. Since she has cut all of these out of her diet she has more energy, has lost weight, and no longer gets canker sores. She didn't know she lacked some energy because she had nothing to compare it to -- she thought she was overweight due to eating fatty foods -- and she thought her canker sores were just something she got since she was a child. But it was all due to allergies to these foods.

There really is a big picture. I am in complete agreement with Dragondawg. While some 'types' of allergies can be seen as something deeper, such as an immune issue, the majority of allergies are mild and alot less serious than cancer and epilepsy. I would breed to a dog with a foodallergy before I bred to one whose close relatives had epilepsy -- and believe me, in my breed, finding a dog without epliepsy nearby is virtually impossible. I am lucky to have a dog with a pedigree that is incredibly healthy compared to the great majority of Belgians these days.