Common allergens are things like corn, wheat, and soy, and sometimes chicken because they eat corn. Talk to your vet or find a pet nutritionist for more info. Dogs with allergies are not cheap to feed, but don't fault good foods like this because your particular dog has a health problem. I would recommend looking at brands like Merrick, Taste of the Wild, TimberWolf Organics, Wellness, Prarie, Innova, Orijen, Weruva. I have heard (but not looked for myself) that these brands often carry grain-free versions and ones that cater to unusual allergies. Additionally, wet foods tend to have less ingredients so they may help you narrow down what exactly your dog is allergic to. :)