What Is It?
An allergy is an immune system reaction to something that is normally harmless. The reaction to the allergen (allergy-triggering substance) results in the release of histamine and other body chemicals. These chemicals cause the symptoms of allergies, which are usually mild but annoying. Examples include the runny nose of hay fever (allergic rhinitis) or the itchy rash of poison ivy.
However, in some cases, the symptoms can be much worse and involve the entire body. Anaphylaxis is the most severe allergic reaction. In anaphylaxis, these immune chemicals cause serious skin symptoms, such as hives and swelling, as well as severe breathing problems and very low blood pressure.