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. Symptoms vary in severity from person to person.
Most environmental allergens contact the skin or are inhaled.
Therefore, most symptoms affect the skin or the breathing passages. Many disorders are associated with, triggered, or worsened by allergies. These include eczema , asthma , and many others. Allergies are common. Tests History is important in diagnosing allergies , including whether the symptoms vary according to time of day or the season and possible exposures such as having a pet in the household.