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Posted Jul 31 2009 11:40am
What Is an Allergy?

Allergies are an abnormal response of the immune system. People who have allergies have an immune system that reacts to a usually harmless substance in the environment. This substance (pollen, mold, animal dander, etc.) is called an allergen.

Allergies are a very common problem, affecting at least two out of every 10 Americans.
What Happens During an Allergic Reaction?

First, a person is exposed to an allergen by inhaling it, swallowing it, or getting it on or under their skin. After a person is exposed to the allergen, a series of events create the allergic reaction
1. The body starts to produce a specific type of antibody, called IgE, to bind the allergen.
2. The antibodies attach to a form of blood cell called a mast cell. Mast cells can be found in the airways, in the intestines, and elsewhere. The presence of mast cells in the airways and GI tract makes these areas more susceptible to allergen exposure.
3. The allergens bind to the IgE, which is attached to the mast cell. This causes the mast cells to release a variety of chemicals into the blood. Histamine, the main chemical, causes most of the symptoms of an allergic reaction.

What Are the Symptoms of an Allergic Reaction?

Common symptoms of an allergic reaction to inhaled or skin allergens include
• Itchy, watery eyes

• Sneezing

• Itchy, runny nose

• Rashes

• Feeling tired or ill

• Hives (a rash with raised red patches)

Other exposures can cause different allergic reactions
• Food allergies. An allergic reaction to food allergens can also cause stomach cramps, vomiting, or diarrhea.

• Insect stings. The allergic reaction to a sting from a bee or other insect causes local swelling, redness, and pain.

The severity of an allergic reaction’s symptoms can vary widely
• Mild symptoms may be almost unnoticeable, just making you feel a little “off.”

• Moderate symptoms can make you feel ill, as if you’ve got a cold or even the flu.

• Severe allergic reactions are extremely uncomfortable, even incapacitating.

Most symptoms of an allergic reaction go away shortly after the exposure stops.

The most severe allergic reaction is called anaphylaxis. In anaphylaxis, allergens cause a whole-body allergic reaction that can include
• Hives and itching all over (not just in the exposed area)

• Wheezing or shortness of breath

• Hoarseness or tightness in the throat

• Tingling in the hands, feet, lips, or scalp

Anaphylaxis is life-threatening and requires immediate medical attention. Symptoms can progress rapidly, so head for the emergency room if there’s any suspicion of anaphylaxis.
Does Everyone Have Allergies?

No, not everyone has allergies. 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.

Symptoms & Types

Sneezing, difficulty breathing, cramps, and vomiting–all are allergy symptoms. Learn the types of allergies, specific allergy symptoms, and emergency warning signs.


Learn the difference between mild and severe allergy symptoms.

The reaction will depend on the body part involved and the severity of the reaction. Here’s what to look for.


Alllergic rhinitis, also known as hay fever, is an allergic response to pollen or other microscopic substances.

Find out what causes hives and how to treat them.

Many people think poison ivy is contagious. Find out if that’s true and the best way to prevent poison ivy and other plant allergies.

A normal reaction to a bee sting is different from a bee sting allergy. Do you know the difference?

An allergy specialist shares her tips on dealing with pet allergies. And she should know. She has a cat, a dog, and many family members who have dog and cat allergies.

Learn about latex allergy symptoms and what to do in case of a severe reaction.

Mold is present in most indoor and outdoor spaces, and in many foods. Find out more about mold allergy symptoms and common food sources of mold.

How do you tell the difference between a normal side effect a drug allergy? Find out here.

This itchy skin rash, common in children and infants, affects some people all their lives. Find out more about eczema treatment and symptoms in WebMD's Eczema Health Center.

How can you tell the difference between pink eye and eye allergies?

Get started here.

Do you get a reaction when you eat nuts, shellfish, or other foods? Learn what’s causing those food allergy symptoms.

Many unsuspecting products contain milk or milk products. Check our list.

If you have an egg allergy, check this food list for potential problems.

Nut Allergy
A nut allergy can become very serious, even fatal, rapidly. This food list will help you identify potential problems.

Some unexpected foods contain fish. If you’ve got a fish allergy, these tips will help keep you safe.

For most people with shellfish allergies, all shellfish must be avoided. Check out this list to learn which foods to avoid.

This allergy is most common among infants. By age two, the majority of children outgrow it. Find out more about the symptoms and how to avoid soy products.

Learn more about photosensitivity – an allergic response to sunlight.

Knowing which ingredients to look for is key to avoiding a wheat allergy. Check this list for unsuspected products that contain wheat.

About 1% of people have a sulfite allergy. This article has a list of foods that may contain sulfites.

Learn more about what triggers allergies in the fall season.

Warning Signs

A severe allergic reaction can be life-threatening. Learn more about the reaction called anaphylaxis so you can be prepared.


Allergies are a main trigger for asthma attacks. See what an asthma attack looks like in the lungs and learn the early warning signs.

How can you tell if your child’s allergies are causing asthma? Find out how to spot the symptoms and learn how asthma is treated in children.

Hay fever allergies can cause sinus blockage and infection. Learn how to spot sinus infection symptoms and how to treat the problem.

How bad are your allergy problems? Find out with WebMD's Allergy and Sinus Health Check.

Diagnosis & Tests

To pinpoint the allergy problem -- and determine the treatment – your doctor will ask questions about your symptoms and habits. You’ll also need various tests.


Diagnosing allergies starts with a doctor’s exam. Learn which questions your doctor may ask so you can be prepared.

Proper testing can make it much easier to live with a food allergy. See a list of the most common food allergies and learn how testing can make your life easier.

If you have a food allergy, you may need to keep a food diary – and remove certain foods from your diet – to determine exactly what you’re allergic to.


An allergy skin test is used to identify the substances that are causing your allergy reactions. Learn more about allergy skin tests, including what happens during the test.

See how blood tests are used to diagnose allergies and learn what can interfere with the test.

Treatment & Care

There are lots of allergy treatment options. Over-the-counter and prescription medications can ease annoying symptoms. Allergy shots also help.


Learn all about the different over-the-counter and prescription medicines that can help ease annoying symptoms.

Mother Nature may be able to provide natural allergy relief. Find out what may help – and what may cause harm.

Find out how antihistamines work, who should not take them, and which foods or drugs may interact with them.

Learn how decongestants work – and who should not use them.

Atrovent nasal spray can help with certain allergy symptoms. Find out if it’s right for you.

Steroid nasal sprays are one of the strongest allergy medications. Find out how they work and how to use them.

Find out when allergy eye drops can help and who should not use them.

These medications are fairly new to the allergy world. Find out if they’re right for you.

This type of medication can help but it’s all in the timing. Find out how to use it for best results.

For some people, allergy shots can mean the end to allergy medication. Find out all you need to know.

Advanced Reading: This article, written for doctors, provides in-depth information on skin allergy treatments.

Get the basics on hay fever treatment.

Advanced Reading: This article, written for doctors, provides in-depth information on food allergy treatments.

Advanced Reading: For in-depth information on latex allergies, read this article written for doctors.


These tips will help you reduce exposure to allergens – at home, work, in the car, outdoors.

These discussion points will help you decide if you’re ready for allergy shots.

If your child has severe allergies, an EpiPen could save his/her life. Learn what you need to know.

If you’ve had a severe allergic reaction in the past, you need to carry an EpiPen. Learn how to use it. It could save your life.

Home Treatment

For tips to help relieve symptoms of an allergic reaction, see home treatment for:

  • A severe allergic reaction . If you have symptoms of a severe allergic reaction, call911or other emergency services immediately.
  • Hay fever symptoms . Take an antihistamine to reduce symptoms of itchy, watery eyes; sneezing; or a runny, itchy nose. Be sure to read and follow any warnings on the label. Don't give antihistamines to your child unless you've checked with the doctor first.

For tips on how to treat dry and irritated skin, see the topic Dry Skin and Itching .

For information on how to treat an insect bite or sting, see the topic Insect Bites and Stings and Spider Bites .

Use the Check Your Symptoms section to evaluate your symptoms if any of the following occur during home treatment:

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