Allergy is an overreaction to the immune system to substances that are otherwise harmless, causing respiratory, skin and systemic diseases. Most common are the respiratory diseases, caused by airborne particles such as grass or tree pollens. For most people these tiny particles are insignificant, but for people with allergy, they can trigger seasonal or chronic respiratory conditions, such as hay fever or allergic asthma.
Allergic diseases are linked through common immunological mechanisms, and co-morbidities are common. The risk of getting a new allergy increases with the number of allergies already present. Disease severity may also increase. For example, rhinitis is a risk factor for asthma, and at least 60% of asthma patients also suffer from allergic rhinitis.
Individuals may react with symptoms upon repeated exposure to the sensitising allergens, and a prolonged exposure to allergens can results in chronic inflammation in the airway. In the beginning airway inflammation may be reversed by allergen avoidance and/or treatment, but chronic inflammation for many years may lead to irreversible destruction of airway tissue.