Allergic Reaction

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Allergic Reaction

Topic Overview

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 doesn't 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.

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

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

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Check Your Symptoms

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Home Treatment

You can use home treatment to relieve symptoms of:

  • Itching or hives. Avoid more contact with whatever you think is causing the hives.
  • A sore throat caused by postnasal drip. Gargle with warm salt water to help ease throat soreness.
  • 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.
  • Allergies that are worse in damp weather. Mold may be the cause of allergies that get worse in damp weather. Mold produces spores that move, like pollen, in outdoor air during warmer months. During winter months, indoor molds can also be a problem.
  • Indoor allergies. Newer, energy-saving homes that are built with double- or triple-paned windows and more insulation keep heat as well as allergens indoors.
  • Allergies to a pet or other animal. When allergies are worse around pets, symptoms may be caused by your pet's dead skin (dander), urine, dried saliva, or hair.
  • Seasonal allergies. These are often caused by exposure to pollen.

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.

Symptoms to watch for during home treatment

Call your doctor if any of the following occur during home treatment:

  • Trouble breathing, wheezing, or tightness in the chest develops.
  • Swelling of the throat, tongue, lips, or face develops.
  • Hives develop or get worse.
  • Swelling gets worse.
  • A skin infection develops.
  • Symptoms have not improved after 2 weeks of home treatment.
  • Symptoms become more severe or more frequent.


To prevent problems with severe allergic reactions:

To prevent seasonal or year-round allergy reactions:

  • Control exposure to outdoor allergens. Limit the time you spend outside during allergy season. This may be the best approach to controlling your symptoms. If you have a seasonal allergy:
    • During the peak of the pollen or mold season, consider taking your vacation in a place that has fewer of these substances.
    • Exercise regularly. Exercise produces adrenaline, a natural way to relieve a stuffy nose. But exercising outdoors may also expose you to more pollen or mold spores.
  • Control exposure to indoor allergens. Newer, energy-saving homes built with double- or triple-paned windows and more insulation keep allergens as well as heat indoors.
    • Use an air conditioner or air purifier with a high-efficiency particulate air (HEPA) filter.
    • Keep the house aired out and dry. Keep the moisture level below 50%. Use a dehumidifier during humid weather.
    • Dust and vacuum 1 to 2 times a week. Use a vacuum cleaner with a HEPA filter, which collects dust-mite particles and pollen. Standard paper bag filters may allow the stirred-up allergens to escape back into the room.
    • Avoid carpet, upholstered furniture, and heavy drapes that collect dust. Vacuuming doesn't pick up dust mites. Remove rugs and wall-to-wall carpeting. Talk with your family about this measure and how this will affect family life. Replace drapes and blinds with roll-down shades or washable curtains.
    • Damp mop the floor once a day. Vacuum the walls, ceiling, closet, and the backs of the furniture once a week to get rid of as much dust as you can.
    • Use baking soda, mineral oil, club soda, or vinegar to clean instead of using harsher cleaning solutions that can produce allergic reactions.
    • Contact a pest control service, if necessary, to get rid of cockroaches. Cockroaches and dead insects may provoke allergic responses if you have allergic asthma.
    • Avoid tobacco smoke, smoke from wood-burning stoves, and fumes from kerosene heaters.
    • Keep air registers closed if there is a pet in the house. This will reduce the amount of animal dander circulating in the house, especially in the bedroom.
    • Repair any water-damaged areas from leaking roofs or basements. These areas can be prime mold-growing areas.
  • Control exposure to animal dander (dead skin or scales from animals). Indoor pets can spread dander and other pet-related allergens such as urine and dried saliva throughout your home. Cats in particular spread dried saliva, but other small animals such as mice and gerbils can spread it too. Hair is often not the problem. Short-haired animals are no less of a problem than long-haired ones.
    • Keep the pet outside of the house or at least out of the bedroom.
    • Bathe your pet once a week.
    • Ask a family member who does not have allergies to clean your pet's litter box.
    • Keep a caged pet, such as a gerbil, outside your home in a garage or shed.
    • Consider finding your pet a new home if your symptoms are severe.
  • Be sure to tell your child's school staff about his or her allergies. This is important so the school knows how to help your child if he or she has an allergic reaction.

Breast-feeding may prevent allergies. Breast-feed your baby for at least 6 months if possible to boost his or her immune system. Feeding only breast milk during the first 6 months of life may reduce the chances that your child will develop food allergies or may decrease the severity of your child's allergies. For more information, see the topic Breast-Feeding.

Preparing For Your Appointment

To prepare for your appointment, see the topic Making the Most of Your Appointment.

You can help your doctor diagnose and treat your condition by being prepared to answer the following questions:

  • What are your allergy symptoms?
  • How long have you had these symptoms?
  • Do you have an idea of what is causing your symptoms?
  • Are your allergies present all year, or do they get better or worse with different seasons?
  • What have you tried at home to decrease your symptoms? Has it helped?
  • What prescription or nonprescription medicines have you tried in the past? What worked and what didn't?
  • What other prescription and nonprescription medicines are you taking?
  • Have you recently gotten a tattoo or body piercing?
  • Do you have any health risks?


ByHealthwise Staff
Primary Medical ReviewerWilliam H. Blahd, Jr., MD, FACEP - Emergency Medicine
Specialist Medical ReviewerH. Michael O'Connor, MD - Emergency Medicine
Last RevisedNovember 12, 2012

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