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What is an Allergy?

What are Allergies?


Updated May 16, 2014

What is an Allergy?

Mast cells release histamine when an allergen is encountered. The histamine response can produce sneezing, itching, hives and watery eyes.

Updated May 16, 2014

What is an Allergy?

The topic of allergies has become routine in our lives, and certainly most everyone has an idea of what an allergy is. Allergies are so common a subject in fact, it seems acceptable to discuss your allergies at a cocktail party with strangers.

An allergy is an abnormal reaction by a person's immune system against a normally harmless substance. A person without allergies would have no reaction to this substance, but when a person who is allergic encounters the trigger, the body reacts by releasing chemicals which cause allergy symptoms. However, just because there is a cause and effect between exposure to a substance and the development of symptoms does not always mean that a person is allergic to that substance. For example, medications have known and expected side effects; a person experiencing one of these side effects is not necessarily allergic to that medication.

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What is Happening During an Allergic Reaction?

During an allergic process, the substance responsible for causing the allergy, or allergen, binds to allergic antibodies present on allergic cells in a person's body, including mast cells and basophils. These cells then release chemicals such as histamine and leukotrienes, resulting in allergic symptoms.

How do Allergies Start?

The allergic person can make allergic antibodies, or IgE, against a variety of allergens, including pollens, molds, animal danders, dust mites, foods, venoms and medications. This occurs through a process called sensitization, where a person’s immune system is exposed to enough of the allergen to make the body produce allergic antibodies to that substance.

With later exposures, that same allergen binds to its corresponding IgE on allergic cells, and the body reacts with symptoms of allergies. Allergic symptoms can vary somewhat with the type of allergen and route of exposure (airborne pollen exposure may cause different symptoms than eating a food to which you are allergic).

Learn how to avoid allergic triggers and avoid specific food allergens.

When and Why do People Develop Allergies?

It is unknown why some people develop allergies and some don’t. Allergies seem to run in families, and in some cases family members can share allergies to specific foods or medications. It appears that the allergic response was once meant to protect the body against parasitic infections, although now seems to be an abnormal response to non-infectious triggers.

Allergies can occur at any time during our lives, but are more common to occur during childhood or young adulthood.

Learn how you can possibly prevent the onset of allergies and asthma.

Next: Find out what symptoms indicate that you may have allergies.


American Academy of Allergy, Asthma and Immunology and National Institute of Allergy and Infectious Diseases.

DISCLAIMER: The information contained in this site is for educational purposes only, and should not be used as a substitute for personal care by a licensed physician. Please see your physician for diagnosis and treatment of any concerning symptoms or medical condition.

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  4. Allergy Basics
  5. What Is an Allergy? - Guide to Understanding Allergies

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