Allergy is caused by hypersensitivity of the immune system towards an allergen in the environment (such as: pollen, dust etc.). Symptoms of allergy can include skin lesions, respiratory signs and gastro-intestinal upset.
External Factor
Allergens invade due to weakening or defect of skin barrier. This happened due to food ingested, ectoparasites infestation, microorganisms, and allergies caused by other substances.