Technically, there is a difference between "food allergy" that causes a histamine reaction (an immediate Ig-E allergy) and a food intolerance like Celiac Disease. A histamine reaction comes from Ig-E (immunoglobulin E, a type of antibody)s in a body pegging the trigger (sea food for instance) as an invader, and swarming to get it out. Ig-Es bring on extremely powerful reactions. This leads to itchy eyes, runny nose, swelling and in severe cases, anaphylaxis (throat swelling shut). Allergy tests look for Ig-E reactions (on skin or in blood) to tiny amounts of potential allergens. Food intolerances are more tricky. They aren't mediated by those antibodies, so they won't show up on an allergy test. As this mother puts it,
After two doctors told me my son didn't have any allergies, the GI doctor finally found the genes for celiac which explained why he had gone from the 95% of weight to the 5% of weight after we introduced gluten (wheat) and dairy into his diet. Technically, he didn't have a life threatening IgE allergy. One doctor even told me, "we have a death and disease model" meaning if you are not going to die from something immediately after eating it, it isn't a problem. But my son was wasting away and his brain was starving for nutrients.
Celiac disease, and other food intolerances, involve immune reactions in the gut, where the food particles are changed into intestine-bombs instead of being digested:
Upon exposure to [gluten] the enzyme tissue transglutaminase modifies the protein, and the immune system cross-reacts with the small-bowel tissue, causing an inflammatory reaction. That leads to a truncating of the villi lining the small intestine (called villous atrophy). This interferes with the absorption of nutrients, because the intestinal villi are responsible for absorption. (Wikipedia)A really succint and helpful summary is here.
Both are examples of the body over-reacting to something that shouldn't trigger a response at all, but they are different enough to require different tests. I appreciated this blog post by a mother whose children have many food intolerances (and she's into Real Food too!). Most people don't know about Celiac disease, and they think that you are being picky if you say you're "intolerant to" something... so I'll probably go on being incorrect but understood and telling waiters and chefs that "I'm allergic to wheat and soy." :)
[[And parents, if you notice that your child has red cheeks, diarrhea, is irritable or sleepless, vomits, gets eczema or a rash after eating any certain food, or has ANY developmental delays, disorders or learning disabilities, or just "fails to thrive" but tests "negative for any food allergies," you might want to try cutting it out, seeing if he improves, and getting a doctor familiar with non-IgE mediated food intolerances. Just because it doesn't show up as an allergy on a blood test doesn't mean that it couldn't possibly be a life-threatening intolerance.]]