Allergies occur when the immune system reacts to an otherwise harmless substance in its environment and your dog can develop allergies or intolerances to substances of vegetable or animal origin at any time in its life allergies are common in small dogs because their digestive tract and skin are very sensitive and they can come into contact with many different substances typical symptoms are itching and inflammation of the skin as well as sickness and diarrhoea royal canin veterinary diet