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This information was reviewed and approved by Carah Santos, MD (12/5/2022).

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 common allergies (such as to 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 many substances. Others are allergic to a few. A person’s allergy is determined by many different factors, including the environment, a person's genes and the way their immune system works.

Allergen Exposure Example

The following story illustrates how allergen exposure can lead to common allergies, such as seasonal pollen allergy or hay fever:

It is fall. The days have started to grow shorter and cooler, and weeds are producing pollen to make seeds for the next growing season. An invisible cloud of pollen floats over a field of ragweed. After being carried by the wind, the pollen settles on the skin of an allergic child and is inhaled into the respiratory tract.

The immune system of an allergic child then produces large numbers of IgE antibodies that uniquely bind to ragweed pollen. These antibodies attach to special cells (i.e., mast cells) in the child's nasal passages and upper respiratory tract. These cells contain strong chemicals called mediators. The best-known mediator is histamine. The child has just become sensitized to ragweed pollen.

When the sensitized child is exposed to the same pollen again, the IgE antibodies that are now attached to the surface of the mast cells bind to the pollen. This sets off an explosion, as histamine and other mediators burst from inside the mast cells. Histamine causes blood vessels to leak fluid, leading to swelling and redness. The eyes become itchy and watery; the nose becomes congested; mucus production increases; and sneezing begins — all classic symptoms of hay fever and re-exposure to an allergen.

Common Allergens

Learn more about the most common types of allergens.

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