What is atopic (allergic) dermatitis or atopy? 

Atopy, or inhalant allergy, is a hypersensitivity reaction to environmental allergens such as pollens, mold spores, dust / dust mites. Exposure to the allergens by inhalation or contact through the skin can trigger the allergy. Even pets who live indoors can be exposed to many allergens: airborne pollens, dust / dust mites, and mold spores. 

Why do pets develop allergies such as atopy?
We (veterinary dermatologists and immunologists) do not have a good answer for this. We believe it is a matter of "genetic susceptibility" and exposure to high levels of allergens at certain times during maturation of the immune system.

What are the signs of atopy?
The most common signs of this allergy are itching of the belly and "arm pits", face rubbing, foot licking, and recurrent ear and skin (staphylococcal) infections. Some pets have sneezing, coughing, and runny eyes. The signs may be seasonal or happen year-round. 

How is atopy diagnosed?
Diagnosing atopy is based on the pet's history of skin problems, lack of response to dietary restriction (see "Food Allergy"), lack of response to external parasite treatment, and results of allergy testing (skin and blood test).
Important point: there is no perfect test for atopy. Pets with no signs of allergies can have positive reactions on skin and blood allergy tests, and sometimes pets with bad allergies have very poor skin and blood test reactions. It is important to eliminate other causes of itching before doing skin and blood allergy tests.

How is atopy treated?
A first step is using medications such as antihistamines, steroids, cyclosporine, and topical medications to minimize itching. Some folks refer to this as "band-aid therapy": it helps control the signs but is not treating the heart of the problem. Unfortunately, this type of therapy often either doesn't work well or has significant side effects. 
The other approach is desensitization (or immunotherapy, allergy serum) based on allergy test results. This involves injections of the offending allergens to try to "re-train" the pet's immune system to be less sensitive to those allergens. It is the best chance to control the problem and minimize use of medications such as steroids. About 70% of dogs and 50-60% of cats respond to this therapy. It takes an average of 6 months to see a response to the injections; some pets respond in 2-3 months; some take 1 year. It is not an "overnight fix". But again, it is the best chance to control the problem and minimize use of medications with side effects.
Important point: Atopy is a life-long condition; either approach (band-aid or desensitization) will be life-long treatment. 

What should I do if I suspect my pet has allergies?

First, we recommend your veterinarian examine your pet. This exam is an important first step in eliminating other causes of itching: fleas, infection, and Sarcoptic mange, for example. They may prescribe medication to control the itching. If the problem recurs, he or she may recommend a restricted diet to rule-out food allergies, allergy testing and/or a consult with a veterinary dermatologist.