Blood Test: Allergen-Specific Immunoglobulin E (IgE)
What It Is
An allergen-specific immunoglobulin E (IgE) blood test is done to check whether a person is allergic to a particular substance.
An allergic reaction occurs when the immune system overreacts to something, often in the environment, that's harmless to most people. To protect the body from this perceived threat, or allergen, the immune system of an allergic person produces antibodies called immunoglobulin E.
IgE antibodies are found mostly in the lungs, skin, and mucous membranes. They cause mast cells (a type of cell involved in the body's immune response) to release chemicals, including histamine, into the bloodstream. It's these chemicals that bring on many of the allergy symptoms that affect a person's eyes, nose, throat, lungs, skin, or gastrointestinal tract.
Because IgE antibodies are unique to each allergen (for example, IgE produced in response to pollen differs from IgE produced after a bee sting), checking for specific variants in the blood can help determine if an allergy is present.
Common allergens that may be tested for by using the allergen-specific IgE test include:
- animal dander
- dust mites
- foods (including peanuts, milk, eggs, or shellfish)
- medications (such as penicillin)
- insect venom (from bee or wasp stings)
- latex (found in certain balloons or hospital gloves)
Your doctor also may order a group of these tests — sometimes called a mini-screen or mini-panel — to look for antibodies against a variety of suspected allergens.
Why It's Done
This test is performed to check for allergies to specific allergens. Doctors may order it when a child has symptoms of an allergy (include hives, itchy eyes or nose, sneezing, nasal congestion, throat tightness, or trouble breathing). Symptoms may be seasonal (as with allergies due to pollen or molds) or year-round (as with pet dander) and can range from mild to severe.
This test is especially useful in children who've had life-threatening reactions to a certain allergen and for whom a skin-prick test would be too dangerous. In some cases, the test may also be used to monitor the effectiveness of allergy treatments, or to see if a child has outgrown an allergy.
No special preparations are needed for this test. On the day of the test, having your child wear a T-shirt or short-sleeved shirt can make things easier for your child and the technician who will be drawing the blood.
A health professional will usually draw the blood from a vein. For an infant, the blood may be obtained by puncturing the heel with a small needle (lancet). If the blood is being drawn from a vein, the skin surface is cleaned with antiseptic, and an elastic band (tourniquet) is placed around the upper arm to apply pressure and cause the veins to swell with blood. A needle is inserted into a vein (usually in the arm inside of the elbow or on the back of the hand) and blood is withdrawn and collected in a vial or syringe.
After the procedure, the elastic band is removed. Once the blood has been collected, the needle is removed and the area is covered with cotton or a bandage to stop the bleeding. Collecting blood for this test will only take a few minutes.
What to Expect
Either method (heel or vein withdrawal) of collecting a sample of blood is only temporarily uncomfortable and can feel like a quick pinprick. Afterward, there may be some mild bruising, which should go away in a few days.
Getting the Results
The blood sample will be processed by a machine, and the results are usually available within a few days.
Elevated levels of specific IgE antibodies may mean an allergy is present. However, the amount of IgE doesn't necessarily predict the severity of the reaction. For this reason, your doctor will interpret the results in comparison with your child's symptoms and other allergy tests.
The allergen-specific IgE test is considered a safe procedure. However, as with many medical tests, some problems can occur with having blood drawn, such as:
- fainting or feeling lightheaded
- hematoma (blood accumulating under the skin causing a lump or bruise)
- pain associated with multiple punctures to locate a vein
Helping Your Child
Having a blood test is relatively painless. Still, many kids are afraid of needles. Explaining the test in terms your child can understand might help ease some of the fear.
Allow your child to ask the technician any questions he or she might have. Tell your child to try to relax and stay still during the procedure, as tensing muscles and moving can make it harder and more painful to draw blood. It also may help for your child to look away when the needle is being inserted into the skin.
If You Have Questions
If you have questions about the allergen-specific IgE test, speak with your doctor.