Allergies - overview
Allergy - allergies; Allergy - allergens
An allergy is an immune response or reaction to substances that are usually not harmful.
Blooming flowers and blossoming trees are signs that spring has arrived. But for some people, those blossoms and blooms are also signs that allergy season is under way. And that means months of sniffling, sneezing, and runny eyes ahead. You may know that you have allergies as soon as you step outside on a spring day, eat peanuts, or pet your dog. Signs of an allergy may include trouble breathing, teary eyes, hives, itching, or vomiting after you come in contact with your allergy trigger. Your doctor can also do allergy tests to find out whether you're allergic, and what triggers your allergies. The most common type of allergy test is a skin test. The doctor puts a small amount of different allergy-causing substances under your skin. Then you wait for signs of a reaction, like swelling or redness. You might also have blood tests to check for chemicals that are related to allergies. So, how are allergies treated? There are a few medicines you can buy at your local drugstore or your doctor can prescribe to treat your allergies. Antihistamines prevent histamine chemicals from triggering allergy symptoms. Decongestants shrink swollen blood vessels throughout your body, including inside your nose, perhaps helping you breathe easier. Steroid drugs reduce swelling and inflammation. And Leukotriene inhibitors block the substances that trigger allergies. If your allergy is really bugging you, your doctor may give you allergy shots. When you take allergy shots over time, eventually they can help your body get used to the substance so you don't over-react to it in the future. Usually you can relieve allergies by taking medicine and avoiding whatever it is that triggers them. But in some people, allergies to insect stings or certain foods, like peanuts, cause a life-threatening reaction. This is called anaphylaxis. And if you have a life-threatening allergic reaction, call immediately for emergency medical help.
Allergies are very common. Both genes and environment play a role. If both your parents have allergies, there is a good chance that you have them, too.
The immune system normally protects the body against harmful substances, such as bacteria and viruses. It also reacts to foreign substances called allergens. These are usually harmless and in most people do not cause a problem.
In a person with allergies, the immune response is oversensitive. When it recognizes an allergen, the immune system launches a response. Chemicals such as histamines are released. These chemicals cause allergy symptoms.
Common allergens include:
- Insect venom
- Pet and other animal dander
Some people have allergy-like reactions to hot or cold temperatures, sunlight, or other environmental triggers. Sometimes, friction (rubbing or roughly stroking the skin) will cause symptoms.
Mostly, the part of the body the allergen touches affects what symptoms you develop. For example:
- Allergens that you breathe in often cause a stuffy nose, itchy nose and throat, mucus, cough, and wheezing.
- Allergens that touch the eyes may cause itchy, watery, red, swollen eyes.
- Eating something you are allergic to can cause nausea, vomiting, abdominal pain, cramping, diarrhea, or a severe, life-threatening reaction.
- Allergens that touch the skin can cause a skin rash, hives, itching, blisters, or skin peeling.
- Drug allergies usually involve the whole body and can lead to a variety of symptoms.
At times, an allergy can trigger a response that involves the entire body.
Exams and Tests
The health care provider will perform a physical exam and ask questions, such as when the allergy occurs.
Allergy testing may be needed to find out whether the symptoms are an actual allergy or are caused by other problems. For example, eating contaminated food (food poisoning) may cause symptoms similar to food allergies. Some medicines (such as aspirin and ampicillin) can produce non-allergic reactions, including rashes. A runny nose or cough may actually be due to an infection.
Skin testing is the most common method of allergy testing:
- The prick test involves placing a small amount of the suspected allergy-causing substances on the skin, and then slightly pricking the area so the substance moves under the skin. The skin is closely watched for signs of a reaction, which include swelling and redness.
- The intradermal test involves injecting tiny amount of allergen under your skin, then watching the skin for a reaction.
- Both the prick and intradermal tests are read 15 minutes after application of the test.
- The patch test involves placing a patch with the suspected allergen on your skin. The skin is then closely watched for signs of a reaction. This test is used to determine contact allergy. It is usually read 48 to 72 hours after application of the test.
The doctor may also check your reaction to physical triggers by applying heat, cold, or other stimulation to your body and watching for an allergic response.
Blood tests that may be done include:
- Immunoglobulin E (IgE), which measures levels of allergy-related substances
- Complete blood count (CBC) during which an eosinophil white blood cell count is done
In some cases, the doctor may tell you to avoid certain items to see if you get better, or to use suspected items to see if you feel worse. This is called "use or elimination testing." This is often used to check for food or medicine allergies.
Every time you walk into your backyard, you start sneezing, sniffling, and feeling like you want to go back indoors. You're pretty sure you have an allergy, but what are you allergic to? Pollen? Grass? Your neighbor's Golden retriever? The only way to know for sure what's making you sneeze is to have allergy tests at your doctor's office. Let's talk about allergy testing. Your doctor may do one or more of several different types of allergy tests to see what's causing your allergies. One is a skin test. It can diagnose allergies to things like mold, pollen, animal fur, insect stings, and foods. With a skin test, your doctor will place a small amount of one substance, or several different substances just under the surface of the skin on your arm or back. You'll feel a little prick when the substances are placed under your skin. After 15 minutes or so, the doctor will look for signs that you're having a reaction. Usually your skin will get red or swollen if you're allergic to something in the test. Another way to test for allergies is to put a patch of the substance on your skin and leave it there for about 2 days, checking the area every day for any sign of a reaction. You may have a blood test. A blood test measures the amount of substances called antibodies that your body produces in response to a certain allergen. If you're allergic to food, you can try avoiding whatever foods you think might be making you sick. This is called elimination testing. Then you add back in each of the foods, one at a time, and look for signs of an allergic reaction. Your doctor may also try to trigger an allergic reaction in the office by having you eat the food or breathe in the substance you think causes your allergy. This is called a challenge test. The one risk to this test is that, if you're severely allergic, you could have a very serious reaction. Your doctor will watch you very closely during this test to make sure you're safe. Allergy tests are usually pretty accurate. But sometimes what bothers you in the real world won't show up on the test. If you have one test and it doesn't find your allergy trigger, your doctor may recommend having another type of test. Don't worry if it takes a while to find the source of your allergies. Your doctor will keep trying different methods until you learn exactly what's making you so miserable.
Severe allergic reactions (anaphylaxis) need to be treated with a medicine called epinephrine. It can be life-saving when given right away. If you use epinephrine, call 911 or the local emergency number and go straight to the hospital.
The best way to reduce symptoms is to avoid what causes your allergies. This is especially important for food and drug allergies.
There are several types of medicines to prevent and treat allergies. Which medicine your doctor recommends depends on the type and severity of your symptoms, your age, and overall health.
Illnesses that are caused by allergies (such as asthma, hay fever, and eczema) may need other treatments.
Medicines that can be used to treat allergies include:
Antihistamines are available over-the-counter and by prescription. They are available in many forms, including:
- Capsules and pills
- Eye drops
- Nasal spray
These are anti-inflammatory medicines. They are available in many forms, including:
- Creams and ointment for the skin
- Eye drops
- Nasal spray
- Lung inhaler
People with severe allergic symptoms may be prescribed corticosteroid pills or injections for short periods.
Decongestants help relieve a stuffy nose. Do not use decongestant nasal spray for more than several days because they can cause a rebound effect and make the congestion worse. Decongestants in pill form do not cause this problem. People with high blood pressure, heart problems, or prostate enlargement should use decongestants with caution.
Leukotriene inhibitors are medicines that block the substances that trigger allergies. People with asthma and indoor and outdoor allergies may be prescribed these medicines.
Allergy shots (immunotherapy) are sometimes recommended if you cannot avoid the allergen and your symptoms are hard to control. Allergy shots keep your body from over-reacting to the allergen. You will get regular injections of the allergen. Each dose is slightly larger than the last dose until a maximum dose is reached. These shots do not work for everybody and you will have to visit the doctor often.
SUBLINGUAL IMMUNOTHERAPY TREATMENT (SLIT)
Instead of shots, medicine put under the tongue may help for grass, ragweed, and dust mite allergies.
Ask your provider if there are any asthma and allergy support groups in your area.
Most allergies can be easily treated with medicine.
Some children may outgrow an allergy, especially food allergies. But once a substance has triggered an allergic reaction, it usually continues to affect the person.
Allergy shots are most effective when used to treat hay fever and insect sting allergies. They are not used to treat food allergies because of the danger of a severe reaction.
Allergy shots may need years of treatment, but they work in most cases. However, they may cause uncomfortable side effects (such as hives and rash) and dangerous outcomes (such as anaphylaxis). Talk with your provider whether allergy drops (SLIT) are right for you.
Complications that may result from allergies or their treatment include:
- Anaphylaxis (life-threatening allergic reaction)
- Breathing problems and discomfort during the allergic reaction
- Drowsiness and other side effects of medicines
When to Contact a Medical Professional
Contact your provider for an appointment if:
- Severe symptoms of allergy occur
- Treatment for allergies no longer works
Breastfeeding can help prevent or decrease allergies when you feed babies this way only for 4 to 6 months. However, changing a mother's diet during pregnancy or while breastfeeding does not seem to help prevent allergies.
For most children, changing the diet or using special formulas does not seem to prevent allergies. If a parent, brother, sister, or other family member has a history of eczema and allergies, discuss feeding with your child's doctor.
There is also evidence that being exposed to certain allergens (such as dust mites and cat dander) in the first year of life may prevent some allergies. This is called the "hygiene hypothesis." It came from the observation that infants on farms tend to have fewer allergies than those who grow up in more sterile environments. However, older children do not seem to benefit.
Once allergies have developed, treating the allergies and carefully avoiding allergy triggers can prevent reactions in the future.
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Last reviewed on: 1/23/2022
Reviewed by: Stuart I. Henochowicz, MD, FACP, Clinical Professor of Medicine, Division of Allergy, Immunology, and Rheumatology, Georgetown University Medical School, Washington, DC. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.