Hypersensitivity means that the immune system overresponds to a particular antigen (foreign material). This is also known as an allergic reaction. The antigens that cause this allergic reaction are known as allergens. Whenever an allergic reaction occurs, there is some type of injury to the body.

Common allergens in animals include flea saliva (flea saliva is injected into the animal during a fleabite), ear mites (insect bites in general) certain foods and pollens. Pollens fall into a special class of allergens called "inhalant allergens."

Scratching (skin itch) and wheezing (bronchitis) are two common symptoms of allergies in animals.

Anaphylactic shock is an extreme allergic condition. The animal develops respiratory symptoms (wheezing and shortness of breath) as bronchioles constrict, accompanied by cardiovascular failure. Decreased blood pressure may be fatal in just a few minutes.

Anaphylactic shock is a medical emergency and requires immediate treatment.