Definition

Allergies are abnormal reactions of the immune system to substances that are otherwise harmless.

Description

Allergies are among the most common medical disorders. By some estimates more than one in every five Americans suffer from some form of allergy. This statistic holds true throughout the world. Allergies are the single most common reason for absence from school, and they are the major cause of hours lost from work.

An allergy is a type of immune reaction. Normally, the immune system protects the body against harmful invaders, like bacteria and viruses. These invaders carry distinctive markers on their outside surface called antigens (pronounced an-TIH-juns). The immune system produces special molecules called antibodies when it finds an antigen. The antibodies attach themselves to the antigens. The antigen-antibody combination sets off a series of changes in the body, which protect the body from infection and disease.

This same sequence of...