Children's Hospital Colorado

Allergic Reactions

What is an allergic reaction?

Allergic reactions occur when the body's immune system responds to a harmless substance as if it were a toxin or a foreign invader. This overactive response can be caused by one of several different types of immune mechanisms and can involve virtually any organ or multiple organs in the body.

The parts of the body that are commonly involved in allergic reactions include the nasal passages, the eyes, the skin and the gastrointestinal tract. Anaphylaxis is the term used to refer to an allergic reaction that is life threatening and involves multiple organ systems.

What causes allergic reactions?

The timing of allergic reactions varies depending upon the immune mechanism involved in causing the reaction. Some allergic reactions occur immediately after a sensitized person is exposed to the allergen, whereas other allergic reactions take hours to days to develop. Skin testing or blood work may help to determine the cause of reaction when symptoms occur soon after exposure to the allergen, but are not always helpful when the allergic reaction occurs hours to days after exposure.

Contact the Allergy and Immunology Center

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Get to know our pediatric experts.

Daniel Searing, MD

Daniel Searing, MD

Allergy and Immunology, Pediatrics

Elena Hsieh, MD

Elena Hsieh, MD

Allergy and Immunology, Pediatrics

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David Fleischer, MD

David Fleischer, MD

Allergy and Immunology