What are allergies?

Allergic disorders affect an estimated 1 in 5 adults and children (40 to 50 million people) and are the sixth leading cause of chronic illness in the United States, according to the Allergy Report from the American Academy of Allergy, Asthma and Immunology (AAAI).

Allergies are the immune system’s inappropriate response to a foreign substance. Exposure to what is normally a harmless substance, such as pollen, causes the immune system to react as if the substance were harmful. Substances that cause allergies are called allergens. Most allergies result from a combination of inheritance (genes) and environmental exposures (pollens, animal dander, etc.). Being exposed to allergens at certain times when the body’s defenses are low or weak, such as after a viral infection, a serious illness, or during pregnancy also may contribute to the development of allergies.

When you come into contact with an inhaled allergen, you may experience nasal/eye symptoms (allergic rhino, conjunctivitis or hay fever) including sneezing, congestion, itchy, watery nose and eyes and/or asthma symptoms such as wheezing, chest tightness, difficulty breathing and coughing.

What is an allergic reaction?

A hypersensitive response, or allergic reaction, is the result of the interaction among the allergen itself, mast cells and immunoglobulin E (IgE). The result is a prompt release of inflammatory chemicals that can cause swelling of tissues, itching, engorgement of blood vessels, increased secretions and bronchospasm (tightening of muscles that surround the airways).

If the allergen is in the air, the allergic reaction will occur in the eyes, nose and/or lungs. If the allergen is ingested, the allergic reaction may primarily occur in the mouth, stomach, and intestines.

What types of allergens cause allergic rhinoconjunctivitis (hay fever)?

The most common allergens are pollens and dust mites. Allergic rhinoconjunctivitis, or hay fever, is the allergic response to pollen. It causes inflammation and swelling of the lining of the nose, as well as the protective tissue of the eyes (conjunctiva). Symptoms include sneezing, congestion, and itchy, watery eyes. Treatment options include over-the-counter and prescription oral and topical medications. These medications include antihistamines, intranasal cromolyn, intranasal steroids, oral anti-leukotrienes, oral decongestants and others. Among the most effective strategies to reduce allergic rhinoconjunctivitis symptoms is avoidance.

Avoiding pollen exposure by staying indoors when pollen counts are high, closing windows and using air conditioning will help reduce symptoms. Avoidance of indoor allergens such as dust mites and mold spores entails measures to reduce indoor humidity. Dust mite exposure can also be reduced by mattress/box spring and pillow encasement, and washing all bedding in hot cycle frequently. Avoiding pets is a challenge for many patients, but can be a very important factor in improving symptoms of allergic rhinoconjunctiviitis and/or asthma. When avoidance measures combined with regular use of medications is not effective, not feasible or not desirable, immunotherapy (allergy shots or a small tablet under the tongue, called sublingual) may be considered.

  • Pollens: Pollens are microscopic particles released into the air by trees, grasses and weeds. When these particles are inspired, people who have inherited the potential to make allergic responses in their immune system may become sensitized. When they are subsequently re-exposed to the same pollen, they may experience symptoms of allergic rhinoconjunctivitis.
  • Dust mites: Dust mites are microscopic insects that live in dust and in the fibers of household objects not frequently laundered, such as pillows, mattresses, carpet and upholstery. Dust mites require warm, humid areas to grow and proliferate. The symptoms of dust mite allergy are similar to those of pollen allergy, and also can be associated with symptoms of asthma such as wheezing and coughing. To help avoid dust mite allergens, try using dust mite covers (air-tight plastic/polyurethane covers) over pillows, mattresses and box springs. Also, remove carpeting or vacuum frequently using a vacuum cleaner with high efficiency filters. Treatment frequently also includes medications and/or allergen immunotherapy (allergy shots).
  • Molds: Molds are parasitic, microscopic fungi with spores that are also released in the air — like pollen. Mold can be found in damp areas, such as the basement or bathroom, as well as in the outdoor environment in grass, leaf piles, hay, mulch, or under mushrooms. Mold spores peak during hot, humid weather.
  • Animal dander: The proteins secreted by sweat glands in an animal’s skin, which are shed in dander, and the proteins present in an animal’s saliva cause allergic reactions in some people. Treatment involves avoiding exposure as much as possible.

Last reviewed by a Cleveland Clinic medical professional on 01/11/2018.


Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy