Allergy Allergens Make an Appointment Refer a Patient Ask a Question Reviewed by Sanny Chan, MD (November 01, 2016) 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. Animal allergy Cockroach allergy Dust mite allergy Insect sting allergy Food allergy Latex allergy Medication allergy Mold allergy Pollen allergy View Allergy Home Allergy Symptoms Allergic Conditions Clinical Trials For more than 100 years, National Jewish Health has been committed to finding new treatments and cures for diseases. Search our clinical trials.