Doing serum IgG tests to determine common alergies are highly unreliable, let alone doing it for crohn's disease.
http://onlinelibrary.wiley.com/doi/10.1111/j.1398-9995.2008.01705.x/abstract;jsessionid=C7D169019CF623EFEF170771106A549B.f01t01?systemMessage=Wiley+Online+Library+will+be+disrupted+9th+Aug+from+10-2+BST+for+essential+maintenance.+Pay+Per+View+will+be+unavailable+from+10-6+BST.
The WHO has forbidden some IgG tests as a dignostic tool for certain diseases. The problems with an antigen response are many, sometimes it is cross reactivity, your body reacts to a protein that is similar even though the test says it's reacting to something else, sometimes the antigen isn't even present but the response is still there, sometimes the reaction is normal since it's just a reaction to the introduction of certain foods. Just because you can find an antigen response to an antigen, doesn't therefor mean that the immune system has mounted a response, or that the adaptive immune response was stimulated by an APC like the study suggests.
The study mentions the ASCA tests and implies that a positive ASCA is the result of an immune response to common baker's yeast. ASCA, which tests for yeast antibodies. There is not one disease that tests positive for ASCA, there are tens of diseases that test positive for ASCA, it's not a reliable test, maybe it's cross reactivity, maybe it's meaningless, removing yeast from diets doesn't help crohn's disease.
Ig tests are popular because they're cheap, and it's easy to get a research grant for them, since there are thousands of companies telling people that disease A B or C is simply a food allergy, and all you have to do is pay them some money, they will check which food you're allergic to, cut the food out of your diet..and voila, magically you'll feel better. And tests don't lie. What they don't tell you is that many of the tests are unreliable.