An allergen is an otherwise harmless substance that the immune system mistakes as being harmful. The immune system's reaction against an allergen is what causes the symptoms of allergy. There are many types of allergens (such as dust mites and food) and conditions caused by them (such as hay fever and eczema).
To become allergic to anything, a person must first have a genetic predisposition for allergy. Then, at least two exposures to an allergen are required: the primary exposure and the secondary exposure.
Some people are allergic to lots of things. Some people are allergic to only one or to very few things. What a person is allergic to is determined by many different factors, including the environment, a person's genes and the way the immune system works.
In this section, learn more about the most common types of allergens.