An allergy is an abnormal and exaggerated immune reaction against foreign substances (allergens).
Actually the body reacts against harmless substances such as pollen or food.
Human and animal allergies are constantly increasing. One possible explanation (hygiene theory) is the large increase of personal hygiene, cleaning, prophylaxis (vaccination, deworming etc.) resulting in inactivity of the immune system, which, being "unemployed", reacts abnormally.
The most common allergic dermatitis in dogs and cats are three (see separate descriptions):
Less common are allergies to the bite of mosquitoes and other insects, and contact allergy.