Toggle: English / Spanish
An allergy is an immune response or reaction to substances that are usually not harmful.
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.
Allergies may make certain medical conditions, such as sinus problems,
, and , worse.
The part of the body the allergen touches affects what symptoms you develop. For example:
- often cause a stuffy nose, itchy nose and throat, mucus, cough, .
- 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 , hives, .
- Drug allergies usually involve the whole body and can lead to a variety of symptoms.
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 medications (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. One type of skin testing is the prick test. It 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. Other types of skin tests include patch testing and intradermal testing. Skin testing may be an option for some young children and infants.
Blood tests that may be done include:
- Immunoglobulin E (IgE), which measures levels of allergy-related substances
- (CBC) called the
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 medication allergies.
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.
Sometimes, a suspected allergen is dissolved and dropped into the lower eyelid to check for an allergic reaction. This should only be done by a health care provider.
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 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 medications 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.
Medications 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 medications. They are available in many forms, including:
- Creams and ointment for the skin
- Eye drops
- Nasal spray
- Lung inhaler
Persons 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. Person 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 and ragweed allergies.
Most allergies can be easily treated with medication.
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.
- 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
Call for an appointment with your health care provider if:
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.
Rossen RD, Birdsall HH. Allergy and immunology. In: Harward MP, ed. Medical Secrets. 5th ed. Philadelphia, PA: Elsevier Mosby; 2012:chap 11.
Wasserman SI. Approach to the person with allergic or immunologic disease. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 257.
- Last reviewed on 5/10/2014
- Stuart I. Henochowicz, MD, FACP, Associate Clinical Professor of Medicine, Division of Allergy, Immunology, and Rheumatology, Georgetown University Medical School, Washington, DC. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.
The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. © 1997- 2013 A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.