Outdoors, mold lives in the soil, on compost, and on damp vegetation. Dust is another common allergen . Mites, including small fragments of dead mites, are the primary causes of dust allergy. Many people are allergic to animals. Most people are not allergic to the animal's fur or feathers.
Some people are allergic to the animal's saliva.
Saliva exposure occurs if the animal licks the person. It may also occur from petting the animal after it has groomed itself or by touching an object that the animal has recently licked or chewed. When an allergen enters the body of a person with a sensitized immune system, it triggers antibody production. Histamine and other chemicals are released by body tissues as part of the immune response. This causes itching , swelling of affected tissues, mucus production, muscle spasms , hives and rashes , and other symptoms.