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What is an Allergy?

An allergy is an abnormal reaction to ordinarily harmless substances, called allergens, that may be:

  • Inhaled
  • Swallowed
  • Come into contact with the skin

Immune System Overreacts

Your immune system is trained to recognize and attack germs, viruses and other foreign substances that can enter your body. The immune system can misread signals and overreact to harmless substances in the environment such as pollensdust or  foods as if they were harmful to the body. Why a person becomes sensitive or allergic is being investigated and the specific cause is still unknown.

Inflammatory Reactions

When an allergen enters the body it triggers the immune system to produce IgE antibodies. These antibodies attach themselves to mast cells, which are usually found in the respiratory and gastrointestinal tracts and the skin.

When the allergen and the IgE antibody combine on the mast cell it causes the cell to release potent chemical mediators such as histamine. Histamine and other various mediators cause the inflammatory reactions that we see as common symptoms of allergy:


When these inflammatory chemicals are released in the  lungs they cause swelling and narrowing of the airways that may lead to coughing, wheezing, and shortness of breath.


When the chemicals are released in the  nose or sinuses you experience a runny nose, watery eyes, sinus pressure or postnasal drip.


On the skin they can produce  hives eczema and various other rashes. In the digestive tract you may experience stomach cramps, or diarrhea.

Many Mast Cells

Occasionally an allergic reaction involves many mast cells and can result in anaphylactic shock, a potentially life threatening condition in which breathing is affected along with a severe drop in blood pressure.

Diagnosis and Treatment

If you suspect you might have allergies see your doctor for a professional  diagnosis and  treatment plan. You may be referred to a doctor that specializes in diagnosing and treating allergies. This doctor will conduct a thorough medical history and examination and may order  allergy skin tests or blood tests to identify the type of allergy you may have.

Contact Information

Allergy and Inflammation
Department of Medicine
Beth Israel Deaconess Medical Center
One Brookline Place, Suite 623
Brookline, MA 02445