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Allergy Blood Tests and Skin Tests
Can you please explain how an allergy test using blood works - for food allergies and inhaled allergies. Is the test reliable?
Thanks Crystal for asking about allergy blood tests.
In order to really understand allergy blood testing and allergy skin testing, you need to know a little bit of how the human immune system works. Humans defend themselves from the outside world by using an elaborate immune defense system. The human immune system is composed of both immune cells, immune proteins [which occur normally in healthy people] and immune antibodies which fight off invading viruses, fungi and bacterial infections.
There are 5 different types of antibodies that normal people make. Antibodies are also referred to as "immunoglobulins". The 5 antibodies are A, G, M, D and E. The 5th type of antibody to be discovered was named IgE, since E is the 5th letter in the alphabet. IgE refers to "immunoglobulun E', and it is this type of antibody immune reaction to something in the outside world that actually produces allergy symptoms.
Allergy antibodies are produced against "allergens". Allergens are anything that can stimulate IgE reactions, and these may be inhaled [pollen allergens], eaten [nuts, eggs] or injected [bee stings] into the body.
A final but important point here. IgE allergy antibodies act like fuses on allergy cells by binding to the specific allergen that induced it to be made in the first place and then, because of this binding, IgE causes the release of histamine from the allergy cells [mast cells].
(I have just finished a slide show for our AllerNet web site which will be available next week.Be sure to see it as it contains some great photos of allergens and allergy cells.)
O.K. Now you are ready to better understand allergy blood tests. A blood sample is collected from a patient with allergy symptoms. The serum, or water portion of the blood sample, is then tested to see if there are any allergy type antibodies which react against known allergen sources such as weeds, grasses, trees, mold spores and/or house dust mites. The allergy blood test is called either an ELISA (Enzyme Linked ImmunoSorbent) or a RAST (Radio AllergoSorbent Test) depending upon what system is being used by the allergy laboratory.
Allergy blood tests measure antibodies, and they do not tell you what will happen. They are a type of "marking" test hat indicates a potential risk of an allergic reaction. Many patients have a positive allergy test to milk and yet can eat milk products. So, think about the blood testing for allergy as a indicator of risk, NOT disease symptoms.
Allergy skin test actually show you what can happen inside of the body. Allergen extracts are placed upon or in the skin of patients, and within 20 minutes an allergic swelling occurs if the patient has allergy antibodies to the allergens placed upon the skin, all of which is due to the release of histamine and other allergy chemicals from allergy cells in the skin.
Allergy skin test are a type of FUNCTIONAL test as compared to the MARKING allergy blood tests.
Both types of allergy tests are reliable, but they give you very different information.
I hope that this helps you to better understand allergy testing and allergy in general.
Steve Kagen, M.D.