It's often the corn in the commercial foods. Most of 'em have it as a filler. I did a lot of research before I started my dog on it, because she also has some immune system problems due to an early bout with Parvo. I don't know how big your dog is, so I can't recommend amounts, but I'll tell you the amounts I feed for my 70 lb. shepherd. BTW, don't try this with cats without a lot of further research. Cats digestive needs are really different than dogs, and they need different vitamins and minerals. I know it can be done, but they are much more difficult.
First, go to a pet store, like Petsmart.Or Petco, or whatever. Get some Dog vitamins. It also helps if you get some Calcium Powder, at a health food store. Dogs have a higher calcium requirement than humans. But if you have a small grinder, you can grind egg shells ( about 1 egg shell per day, for a 50-75 lb. dog). That's really the only extras you have to buy.
Our recipe for a medium sized dog: (this is per day)
After the first few days, it's very easy and quick to do, and I noticed a huge difference with my dog. Her coat is so shiny, and she is very healthy-none of the immune system problems that had been plaguing her. Halve the recipe for smaller dogs, and 1 1/2 -2 times for larger dogs.
I don't really worry about measuring everything out anymore-just approximations and works out great. Also another note, when trying it out, it's easier to start out slower. If you just dump the dog on a new diet, it will be hard on their digestive system, and they will have diarrhea for a week or 2-though I'm lazy and I did just that, with the aforementioned symptoms:(. I also use some extra supplements for her immune system, and Demodex, but I'm assuming yours is healthy except for the allergies, so don't worry about anything except a decent quality canine vitamin. If you need anything else, just ask, and I'll try to answer as best I can.
Talia