Allergy caused from allergic response can range from mild or moderate to severe, including symptoms like swelling of the face and tongue, rash called "hives" (like nettle rash), breathing difficulties, runny nose and eyes, swelling of the throat, abdominal pain and bowel disturbances, nausea and vomiting and could to life threatening collapse (anaphylaxis).
Cat allergy, in a recent report, affects between six and ten million people (that's approximately two percent of the population!) in the United States or exhibit symptoms.