An allergic response develops when the immune system "overreacts" to a particular foreign material, which is called an allergen. Common allergens are pollens, molds, food proteins, and flea saliva. In animals, allergens often enter the body through breathing or eating, or via flea bites. Allergens can also enter directly through the skin; a common example in people is a reaction to poison ivy, but such contact allergies are unusual in pets.
When an allergen enters the bloodstream, the body produces specific antibodies that "target" the foreign material. This causes specialized white blood cells to release many substances that result in an allergic response. One of the most common of these substances is histamine, which is why antihistamines (eg, diphenhydramine) are sometimes used for treatment.
When people breathe in allergens such as pollen, they typically get hay fever. When pets breathe in these allergens, instead of a stuffy nose or runny eyes, they get a condition called atopy, which results in skin irritation and itching. Many allergic pets begin to show signs when they are as young as 6-7 months, and almost all show signs by the time they are 3-5 years old. Typically, the skin irritation and itching begins as a seasonal problem in the spring or summer, but many pets eventually become allergic year round. The location of the irritation and itching tends to vary depending on the cause. Inhaled allergens usually cause problems around the face, feet, and "arm pits," although other areas of the body can be affected. Food allergies usually cause skin irritation around the head and neck. Flea allergy often develops first in areas where fleas are commonly found, especially the rump, tail, and groin areas.
Allergies can cause your pet to lick and scratch incessantly, leading to skin rashes and serious infections. You can help your veterinarian identify the cause of the problem by keeping track of when the problem started and how (if) it changes over time. Once your veterinarian establishes the cause of the allergy, he or she can determine the best treatment for the allergy. Antihistamines and corticosteroids are often used to combat itching and inflammation. Your pet may need a restricted or special diet to avoid exposure to food allergens. In some cases, your veterinarian may recommend a series of shots meant to gradually desensitize your pet to the offending allergen(s).