Wednesday, September 30, 2009

What's an IgE? An Introduction....

I've been wanting to post about IgE numbers for a while, but it involves science and medicine, neither particularly strong subjects for me.  I'm on the opposite end of the spectrum with interests centering around art and design and creative thinking.  So, why even tackle the subject?  Unfortunately, it just keeps coming up.

In conversations, parents of allergic children almost always get around to telling each other what their child's IgE number and/or Class is.  It's pretty much the only definitive description we can say about an otherwise very uncertain physical condition.  My daughter is allergic to peanuts.  Her IgE number is 14.10 kU/L, and she is a Class 3.  The number is up from the last time we had her tested when she was 4.  It does not mean that she is becoming "more allergic".  Her IgE number only tells the likelihood of her having a reaction.  At a 14, I know she has a 95% chance of having a reaction if she ingests any peanut protein.  What I don't know is when or if she'll have a reaction or to what degree of severity.  I don't know if the reaction will only need an antihistamine or require the dreaded EpiPen.  I don't even know how she got her allergy or if she'll have it her entire life.  That IgE number is the only known in a very scary world of unknowns.  We parents hold onto that number.  But what is it really?

There are 3 ways to determine if someone has an allergy.  The most definitive, most expensive and most risky is a food challenge.  The patient starts with a tiny amount of food and continues to ingest until the first sign of a reaction.  If there is a reaction, then the person is allergic.  If not, they're not allergic.  Usually the amount of food is so small that a patient might just break out in hives or maybe vomit, but there is the risk of  anaphylaxis.  At Duke, whenever a food challenge is administered (at least in the clinical trials) an IV is inserted as a precaution.  Abigail is not scheduled for a food challenge until spring of 2010.

Another allergy test method is a skin prick test.  In this test, the skin is pricked and the allergen in question is applied.  If there is an allergy, the prick area swells and the skin around it gets red and itchy (almost like a mosquito bite). The test can be used for both environmental and food allergens.  While this test is uncomfortable and somewhat subjective, it is less expensive and more reliable than the next method which is the blood test.  About.com does a good job describing and comparing both of these tests in layman's terms.  According to the site, blood tests are becoming more useful in determining food allergies because they measure the specific amount of allergic antibody to a food.  The downside of blood tests, however, is they tend to give more false positives and negatives, and may indicate an elevated antibody, i.e., an allergy, which may or may not necessarily equate to an allergic reaction.  It's from the blood test that the IgE numbers and Classes are assigned. 

Blood Test (RAST, CAP-FEIA, ImmunoCAP)

The RAST test or radioallergosorbent test detects the amount of IgE that reacts specifically with a suspected or known allergen (Wikipedia).  It's an older test and many labs are using a non-radio active test like the CAP-FEIA or ImmunoCAP.  Duke University Medical Center uses the CAP-FEIA.  It's important that you know which test your doctor is using, but I'll get to that later.

So what is an IgE?  I'm using a definition from kidshealth.org for Immunoglobulin E (IgE).  It's the simplest definition I can find online.  "IgE is a type of protein in the body called an antibody. As part of the immune system, it plays a role in allergic reactions. When a person is allergic to a particular substance, such as a food or dust, the immune system mistakenly believes that this usually harmless substance is actually harmful to the body. In an attempt to protect the body, IgE is produced by the immune system to fight that particular substance. This starts a chain of events leading to allergy symptoms."  For example, airway constriction in asthma, local inflammation in eczema, increased mucus secretion in allergic rhinitis and potentially fatal drop in blood pressure as in anaphylaxis (again from Wikipedia).   However, perhaps our nurse last week at Duke explained it best.  As she described it to Abigail....the peanut soldiers think they need to fight peanuts but we're training them not to.

In a discussion with Dr. Kim at our last visit to Duke, I learned how the IgE number is determined.  Peanut protein is adhered to the inside of a test tube.  The patient's blood is then poured into the tube, left to sit for a controlled amount of time and then shaken.  The peanut antibodies in the blood will stick to the peanut protein.  In other words, the antibodies in the blood "attack" the protein in the tube much like it does in the patient's body.  The extra blood is then washed off and the blood that is attached to the protein is measured.  That amount is the IgE number.  The Class is something assigned by the particular lab running the sample.

I've found a RAST scale that I want to post.  I also want to discuss a warning about how the big laboratories use different standards when reading the results.  There's also more on the topic about what the IgE numbers actually mean, i.e., chance of out-growing the allergy and chance of having a reaction.  But, I can just picture eyes glazing over with the amount of information thus far.  So, until next time....

1 comment:

Julie said...

Thanks for all the info. I learned a lot about allergies. I have only ever had the skin prick test but every time I move and have to be retested I found out I am allergic to more things! I also find that every state and every doctors office test and do thing differently. There seems to be no standard.