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IgG blood testing for delayed food allergies?

Q. A family member had a blood test called IgG to check for any delayed allergies. It showed milk and eggs to be a severe, but delayed allergy (no skin reaction). Is there a blood test that can check if she has a delayed allergy to other birds’ eggs (i.e. turkey, duck, quail, etc.) and other animals’ milk (i.e. goat milk, sheep milk, or maybe unpasteurized raw cow milk, etc.)?



A. In IgG testing, the blood is tested for IgG antibodies instead of being tested for IgE antibodies (the antibodies associated with food allergies). IgG is a “memory antibody”.
When you have a blood test to query response to an immunization, this is also IgG testing. A common example is a “Rubella titer”.
In the context of food, IgG signifies memory through exposure to a food. Because a normal immune system should make IgG antibodies to foreign proteins (to include foods), a positive IgG test to a food is a sign of a normal immune system, and suggests tolerance or “memory” of the food rather than food allergy. Therefore, IgG testing is not recommended for evaluation of food allergies.
If the patient has previously eaten the food (milks, eggs), he or she would likely have IgG to the food.



Q. My son was diagnosed with peanut allergy by screening blood testing when he was 18 months old (done for a family history of food allergy in first cousins) but he never had a major reaction to peanut before the diagnosis, and nothing has happened since. He is now 5 years old. He has had cookies that were made in a facility where peanuts are present, without any reaction. He recently had a negative skin test for peanut and his last blood test level was 2.3. I was told that my son should continue to avoid peanuts. However, I recently read about a new kind of blood test for peanut allergy, and I am wondering if this test could be helpful for my son?



A. Peanut allergy seems to be on the rise in the US over the past decade. While there are some promising treatments being researched, the current standard of care is complete avoidance of peanut. Because this restriction can have such a major impact on everyone involved, it is very important that you receive an accurate diagnosis. Peanut allergy affects most areas of a person s life, from the home setting, to play dates, to school, to dining out and beyond.The most important factor in making an accurate diagnosis of peanut allergy is the actual history of the type of reaction that occurred upon consuming a peanut. Specific IgE blood tests (like ImmunoCAP, a common test) and skin prick tests are used in combination with the clinical history to make a diagnosis. In some cases an allergist-supervised oral food challenge is recommended, and this is, in fact, considered the gold standard for accurate diagnosis of allergy to peanut. (This same approach is applied to any possible IgE-mediated food allergy, not just to peanut.)One problem that allergists face is that some people do not have a clear-cut history of reaction to peanut. Situations that allergists see frequently include:In these cases, allergists will typically perform a skin prick test to gain more information. If the skin test is negative, a specific IgE blood test such as ImmunoCAP test can be ordered to gain more information. If the test comes back negative (meaning complete absence of peanut-specific IgE or a very low positive result with no history of anaphylaxis or other serious reaction), an allergist will often proceed to an oral food challenge in the office to confirm the test results,However, if the first blood test comes back positive, yet the clinical history is vague or indicates a mild reaction history, a new test, called the peanut "component test", can be ordered to gain more information in this situation. This component test - the one you are asking about - can determine which specific peanut proteins are triggering the positive test results. It is important to note that there are many smaller protein fragments that make up a whole peanut. Thus, when a person reacts to peanut, he or she may be responding to one or more different protein fragments in the peanut. Determining which of these protein pieces are causing the reaction is important, as some (scientific names Ara h 1 , Ara h 2 , and Ara h 3 ) carry more risk than others. Thus, if these specific tests are negative, there is less risk, and if positive, there is more risk. This will help guide whether an oral food challenge would still be okay (despite the positive initial peanut blood test).Given your son s unclear history of reaction to peanut, we would recommend that you speak to your allergist about the peanut component test and a possible oral food challenge depending on the results of the test. The information gained from the test will be helpful to you, either way!

See the original questions and answers here at the American College of Allergy, Asthma & Immunology


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