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 (insect stings, injected drugs) the cardiovascular system may also be affected. Next we describe the main allergic conditions: