What Are Allergies?
Allergies are an overreaction of the immune system. People who have allergies have a hyper-alert immune system that overreacts to a substance in the environment called an allergen. Exposure to what is normally a harmless substance, such as pollen, causes the immune system to react as if the substance is harmful.

Allergies are a very common problem, affecting at least 2 out of every 10 Americans.

What Happens During an Allergic Reaction?

When a person with a hyper-alert immune system is exposed to an allergen, a series of events takes place:

The body starts to produce a specific type of antibody, called IgE, to fight the allergen.
The antibodies attach to a form of blood cell called a mast cell. Mast cells are plentiful in the airways, and in the GI tract where allergens tend to enter the body.
The mast cells explode releasing a variety of chemicals including histamine, which causes most of the symptoms of an allergy, including itchiness or runny nose.
If the allergen is in the air, the allergic reaction will occur in the eyes, nose, and lungs. If the allergen is ingested, the allergic reaction will occur in the mouth, stomach, and intestines. Sometimes enough chemicals are released from the mast cells to cause a reaction throughout the body, such as hives, decreased blood pressure, shock, or loss of consciousness. This severe type of reaction is called anaphylaxis and can be life-threatening.
What Are the Symptoms of Allergies?

Allergy symptoms can be categorized as mild, moderate, or severe (anaphylactic).

Mild reactions include those symptoms that affect a specific area of the body such as a rash or hives, itchy, watery eyes, and some congestion. Mild reactions do not spread to other parts of the body.
Moderate reactions include symptoms that spread to other parts of the body. These may include itchiness or difficulty breathing.
A severe reaction, called anaphylaxis, is a rare, life-threatening emergency in which the body's response to the allergen is sudden and affects the whole body. It may begin with the sudden onset of itching of the eyes or face and within minutes progress to more serious symptoms, including varying degrees of swellings that can make breathing and swallowing difficult, abdominal pain, cramps, vomiting, and diarrhea. Mental confusion or dizziness may also be symptoms, since anaphylaxis causes a quick drop in blood pressure.
Does Everyone Have Allergies?
No. Most allergies are inherited, which means they are passed on to children by their parents. People inherit a tendency to be allergic, although not to any specific allergen. When one parent is allergic, their child has a 50% chance of having allergies. That risk jumps to 75% if both parents have allergies.

Reviewed by the doctors at The Cleveland Clinic Department of Pulmonary, Allergy and Critical Care Medicine.

Edited by Charlotte E. Grayson, MD, Feb. 2004, WebMD.

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