Dog allergies are caused by an over-reaction of the immune system to a foreign substance, known as an allergen. Exposure to the allergen, usually on multiple occasions spanning months to years, sensitizes the immune system, and subsequent exposure to the same or related allergen causes an over-reaction. In the case of dog allergies, the allergens are proteins found in dog hair, dander, saliva, and urine. These allergens can trigger an allergic reaction when they come into contact with the membranes that line the eyes and nose or when they are breathed into the airways.

The most common symptom associated with dog allergies is itchy skin, either localized (in one area) or generalized (all over the body). In other cases, the allergic symptoms affect the digestive system, resulting in vomiting and diarrhea. Other symptoms of dog allergies include sneezing, runny nose, itchy, red or watery eyes, nasal congestion, itchy nose, roof of mouth or throat, postnasal drip, cough, facial pressure and pain, frequent awakening, raised, red patches of skin (hives), eczema, and itchy skin.

Dog allergies can be diagnosed through a skin-prick test, where a small amount of an extract of dog allergen is placed on the skin, and the skin is pricked with a small, sterile probe, allowing the liquid to seep under the skins surface. The patient is then monitored for swelling and redness or other signs of a reaction, signaling an allergy. The best way to prevent dog allergies is to avoid or reduce exposure to dogs as much as possible.