Allergic conditions are very frequent, and have been increasing in the last 20-30 years. They affect around one in every four persons in developed countries. There is a very high variability regarding frequency and severity of symptoms. The frequency ranges from just a few days a year to everyday symptoms. Likewise, severity ranges from very mild symptoms, which can go unnoticed, to disabling or life threatening reactions, and even death.
Allergy is a systemic condition. It affects the whole organism, although the symptoms mainly appear in organs that are in contact with the environment: the skin, and the respiratory and digestive systems. When the allergens enter the bloodstream  the cardiovascular system may also be affected.