Allergic Reaction

Allergies are an overreaction of the body's natural defense system that helps fight infections (Reference immune system Opens New Window). The immune system normally protects the body from viruses and bacteria by producing Reference antibodies Opens New Window to fight them. In an Reference allergic reaction Opens New Window, the immune system starts fighting substances that are usually harmless (such as Reference dust mites Opens New Window, 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 (Reference allergen Opens New Window). 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 Reference hives Opens New Window 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 (Reference anaphylaxis Opens New Window) occurs, when Reference allergies cause other problems (such as nosebleeds, ear problems, wheezing, or coughing), or when home treatment doesn't help.

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

Types of allergies

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

• Reference 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 Reference Food Allergies.
• Reference 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.
• Reference Allergies to insect venom Opens New Window. 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 Reference Allergies to Insect Stings.
• Reference 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 Reference Allergy to Natural Rubber (Latex).
• Allergies that develop from exposure to a particular inhaled substance in the workplace. These are called Reference occupational asthma.
• Allergies to cosmetics, such as artificial nails, hair extensions, and henna tattoos.

Reference 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.

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

Reference Check your symptoms to decide if and when you should see a doctor.

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