Dust, cats, peanuts, cockroaches. An odd grouping, but one with a common thread: they cause allergies — a major source of illness in the U.S. Up to 50 million Americans have some type of allergy.

An allergy is an overreaction of the immune system to a substance that's harmless to most people. But in someone with an allergy, the body's immune system treats the substance (called an allergen) as an invader and reacts inappropriately, resulting in symptoms that can be anywhere from annoying to possibly harmful to that person.

In an attempt to protect the body, the immune system of the allergic person produces antibodies called immunoglobulin E (IgE). Those antibodies then cause mast cells and basophils (allergy cells in the body) to release chemicals, including histamine, into the bloodstream to defend against the allergen "invader." It's the release of these chemicals that causes allergic reactions, affecting a person's eyes, nose, throat, lungs, skin, or gastrointestinal tract as the body attempts to rid itself of the invading allergen. Future exposure to that same allergen (things like nuts or pollen that you can be allergic to) will trigger this allergic response again. This means every time the person eats that particular food or is exposed to that particular allergen, he or she will have an allergic reaction.

Signs and Symptoms

The type and severity of allergy symptoms vary from allergy to allergy and person to person. Allergies may show up as itchy eyes or an itchy nose, sneezing, nasal congestion, throat tightness, trouble breathing, and even shock (faintness or passing out).

Symptoms can range from minor or major seasonal annoyances (for example, from pollen or certain molds) to year-round problems (from allergens like dust mites or food). Allergies to dust mites are common to the eastern parts of the United States, but not in areas of high-altitude and low humidity (for example, like Colorado). Because different allergens are more prevalent in different parts of the country and the world, allergy symptoms can also vary, depending on where you live. For example, peanut allergy is unknown in Scandinavia, where they don't eat peanuts, but is common in the United States, where peanuts are not only a popular food, but are also found in many of the things we eat.

Airborne Allergy Symptoms

Airborne allergens can cause something known as allergic rhinitis, which occurs in about 15% to 20% of Americans. It develops by 10 years of age and reaches its peak in the early twenties, with symptoms often disappearing between the ages of 40 and 60.

Symptoms can include:

• sneezing
• itchy nose and/or throat
• nasal congestion
• coughing

These symptoms, often accompanied by itchy, watery, and/or red eyes, are called allergic conjunctivitis. (When dark circles are present around the eyes, they're called allergic "shiners.") Those who react to airborne allergens usually have allergic rhinitis and/or allergic conjunctivitis. If a person has wheezing and shortness of breath, the allergy may have progressed to become asthma.

About Anaphylaxis

In rare instances, if the sensitivity to an allergen is extreme, an adult or child may experience anaphylaxis (or anaphylactic shock) — a sudden, severe allergic reaction involving various systems in the body (such as the skin, respiratory tract, gastrointestinal tract, and cardiovascular system).

Severe symptoms or reactions to any allergen, from certain foods to insect bites, require immediate medical attention and can include:

• difficulty breathing
• swelling (particularly of the face, throat, lips, and tongue in cases of food allergies)
• rapid drop in blood pressure
• dizziness
• unconsciousness
• hives
• tightness of the throat
• hoarse voice
• lightheadedness

Anaphylaxis can happen just seconds after being exposed to a triggering substance or can be delayed for up to 2 hours if the reaction is from a food. It can involve various areas of the body.

Fortunately, though, severe or life-threatening allergies occur in only a small amount of individuals. In fact, the annual incidence of anaphylactic reactions is small — about 30 per 100,000 people — although those with asthma, eczema, or hay fever are at greater risk of experiencing them. Most anaphylactic reactions — up to 80% — are caused by peanuts or tree nuts.

Diagnosing Allergies

Some allergies are fairly easy to identify because the pattern of symptoms following exposure to certain allergens can be hard to miss. But other allergies are less obvious because they can masquerade as other conditions. If you or your child has cold-like symptoms lasting longer than a week or two or develops a "cold" at the same time every year, consult your doctor, who will likely ask questions about the symptoms and when they appear. Based on the answers to these questions and a physical exam, the doctor may be able to make a diagnosis and prescribe medications or may refer you to an allergist for allergy skin tests and more extensive therapy.

To determine the cause of an allergy, allergists usually perform skin tests for the most common environmental and food allergens. These tests can be done in infants, but they're more reliable in kids over 2 years old.

A skin test can work in one of two ways:

1. A drop of a purified liquid form of the allergen is dropped onto the skin and the area is pricked with a small pricking device.
2. A small amount of allergen is injected just under the skin. This test stings a little but isn't extremely painful. After about 15 minutes, if a lump surrounded by a reddish area appears (like a mosquito bite) at the injection site, the test is positive.

If reactions to a food or other allergen are severe, a blood test may be used to diagnose the allergy so as to avoid exposure to the offending allergen. Skin tests are less expensive and more sensitive than blood tests for allergies. But blood tests may be required in children with skin conditions or those who are extremely sensitive to a particular allergen.

Even if a skin test and/or a blood test shows an allergy, an adult or child must also have symptoms to be definitively diagnosed with an allergy. For example, a toddler who has a positive test for dust mites and sneezes frequently while playing on the floor would be considered allergic to dust mites.