What Is an Allergy?

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Allergies are an overreaction of the body's natural defense system that helps fight infections (immune system). The immune system normally protects the body from viruses and bacteria by producing antibodies to fight them. In an allergic reaction, the immune system starts fighting substances that are usually harmless (such as dust mites, pollen, or a medicine) as though these substances were trying to attack the body. This overreaction can cause a rash, itchy eyes, a runny nose, trouble breathing, nausea, and diarrhea.

An allergic reaction may not occur the first time you are exposed to an allergy-producing substance (allergen). For example, the first time you are stung by a bee, you may have only pain and redness from the sting. If you are stung again, you may have hives or trouble breathing. This is caused by the response of the immune system.

Most people will have some problem with allergies or allergic reactions at some point in their lives. Allergic reactions can range from mild and annoying to sudden and life-threatening. Most allergic reactions are mild, and home treatment can relieve many of the symptoms. An allergic reaction is more serious when severe allergic reaction (anaphylaxis) occurs, when allergies cause other problems (such as nosebleeds, ear problems, wheezing, or coughing), or when home treatment does not help.

Allergies often occur along with other diseases, such as asthma, ear infections, sinusitis, and sleep apnea. For more information, see the topic Allergic Rhinitis.

Types of allergies

There are many types of allergies. Some of the more common ones include:

Food allergies, which are more common in children than adults. Food allergies are most common in people who have an inherited tendency to develop allergic conditions. These people are more likely to have asthma and other allergies. For more information, see the topic Food Allergies.
Medicine allergies. Many prescription and nonprescription medicines can cause an allergic reaction. Allergic reactions are common and unpredictable. The seriousness of the allergic reaction caused by a certain medicine will vary.
Allergies to insect venom. When you are stung by an insect, poisons and other toxins in the insect's venom enter your skin. It is normal to have some swelling, redness, pain, and itching at the site of a sting. An allergic reaction to the sting occurs when your body's immune system overreacts to the venom of stinging insects. For more information, see the topic Allergies to Insect Stings.
Allergies to animals, which are more likely to cause breathing problems than skin problems. You may be allergic to your pet's dead skin (dander), urine, dried saliva, or hair.
Allergies to natural rubber (latex). Some people develop allergic reactions after repeated contact with latex, especially latex gloves. For more information, see the topic Allergy to Natural Rubber (Latex).
Allergies that develop from exposure to a particular inhaled substance in the workplace. These are called occupational asthma.
Allergies to cosmetics, such as artificial nails, hair extensions, and henna tattoos.

Seasonal allergies show up at the same time of the year every year and are caused by exposure to pollens from trees, grasses, or weeds. Hay fever is the most common seasonal allergy.

Allergies that occur for more than 9 months out of the year are called perennial allergies. Between 20 and 40 million people in the United States are affected by perennial allergies.

Year-round symptoms (chronic allergies) are most likely to occur from exposure to animal dander, house dust, or mold.

Use the Check Your Symptoms section to decide if and when you should see a doctor.

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