What Are the Different Causes of an Allergic Reaction?

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  • Written By: Amanda R. Bell
  • Edited By: E. E. Hubbard
  • Images By: Ajkkafe, Alain Wacquier, Wiml, Greg Friese, Defun, Mariec
  • Last Modified Date: 12 January 2019
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Allergic reactions are caused by the body’s immune system treating a substance, referred to as an allergen, as dangerous, despite the fact that it is not. The most common causes of an allergic reaction are breathing in or ingesting a substance to which the immune system is sensitive. Physical contact with an allergen or having one injected may also be one of the causes of an allergic reaction.

The primary function of the immune system is to protect the body from dangerous viruses, bacteria, and other substances. Sometimes, however, the immune system views a technically harmless material as dangerous and begins to create antibodies specifically meant to protect the body from the substance. These antibodies lie dormant after the first contact with the allergen, and then release histamines and other chemicals into the body once they are exposed to the allergen in the future. The exposure to the allergen and the subsequent rush of chemicals are the primary causes of allergic reactions. Whether the immune system responds to any number of common allergens, such as animal dander, peanuts, or pollen, depends entirely on each individual person; however, early exposure to common allergens or genetics may increase a person’s susceptibility to developing an allergy to the item in the future.


Allergens can come in contact with the body in several different ways. The most common causes of an allergic reaction are breathing or inhaling one. This is typically the cause of seasonal allergies, where small particles of pollen, ragweed, or other common allergens are inhaled simply by breathing. For those sensitive to mold or animal dander, inhaling small spores or hair can also cause a reaction.

Food allergies are one of the most common causes of an allergic reaction. These typically occur early in life, when a child’s immune system overreacts to a new food item causing sensitivity to the item. Each subsequent ingestion of the allergen will typically be one of the causes of an allergic reaction. In some cases, food allergies are so severe that a person who simply is exposed to unseen particles of the food item may experience a moderate to severe reaction.

Physical contact with an allergen can also cause an overreaction of the immune system. This allergic reaction may result in contact dermatitis, where a rash develops on the skin, or can cause an allergic response in other areas of the body. Latex and plant oils are the most common causes of allergic reactions in these cases, although an individual’s immune system can theoretically overreact to nearly any substance it encounters.

One of the causes of allergic reactions may also be injections of an allergen in the system. This can occur with medications or vaccines where an allergen is used as ingredient or preservative in the injection. Stings or bites from other animals may also be causes of allergic reactions when the body responds to the venom in the bite, and, in some cases, can be extremely severe.



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