Dogs itch due to a reaction inside their bodies to an allergen. What are the different types of allergens? One of the most common is environmental allergies that cross the skin’s protective barriers. These can be trees, grasses, pollen, molds, weeds, and even dust mites in the house. Another common allergen is parasitic allergies like fleas or mites while even internal parasites can cause skin irritation. The final, and probably biggest, category is food allergies. Animals can become allergic to proteins in their food such as chicken, beef, fish, and more. See below for the Top 24 most common food allergens as reported by a food allergy test called NutriScan. An elimination diet is the best way to determine what your dog can and cannot eat but it is helpful to know which foods could be triggering the itch.