Food allergies, and the fact that people think they have them, are all over the news because of a big JAMA study that recently came out. I wanted to add my two cents and identify what the study actually assessed. JAMA reviewed 72 studies that used the skin prick test, food-specific IgE blood tests, and elimination diets. They were looking for immediate food reactions because those are generally what allergists use for identifying allergies.
The different testing options
First, I have never really liked skin prick tests. I guess if you are really sloppy when you eat then a skin prick test helps, but otherwise they don’t have great correlation to food issues in people who eat food.
Next, it is great to identify people with immediate and anaphalaxic reactions. However, IgE is not the only immunoglobulin, there are many others such as IgA (secreted at the mucus membrane), IgG (associated with delayed food reactions and a blocker of IgE), IgM, and IgD—it’s not just IgE we need to be concerned about. IgG makes up 75% of the plasma concentration, while all the others combined make up the other 25%. A recent NY Times article on allergies also agreed with the benefits of testing IgG.
Lastly, though I like the use of food elimination diets—they didn’t find many in the 72 studies reviewed. These diets can be hard to fully implement and assess in clinical studies so they are not often used. They do conclude by saying: "The evidence for the prevalence and management of food allergy is greatly limited by a lack of uniformity for criteria for making a diagnosis." I agree.
Why do we use IgG at Metametrix?
Reactions to food are classified into two major categories, IgE mediated (allergic) and non-IgE mediated food reactions. The IgE food reactions are primarily Th2 mediated and generally occur immediately after exposure. IgG reactions often cause different kinds of symptoms such as bloating or sluggishness after eating, GI symptoms, dark circles under the eyes, chronic post nasal drip or sinus congestion—and these symptoms can occur hours or days after eating the offending food.
IgG reactions are primarily a Th1 response, which includes increased production of interferon-gamma (INF-gamma), tumor necrosis factor-alpha (TNF-alpha), and other pro-inflammatory cytokines whose reactions are associated with delayed response to foods.
If you would like a more detailed explanation of how I see the difference between IgG and IgE, you can visit a past newsletter (PDF): May, 2009 newsletter Volume 4, Issue 5