Here is an article written by Dr. Alessio Fasano, one of the world's leading experts on Celiac Sprue, an autoimmune disease that causes poor absorption of vitamins, minerals and other nutrients in the intestines. The gist of the article is that Celiac Disease, like other autoimmune diseases, occurs in the presence of 3 factors:
1. Exposure to an environmental factor (in this case, gluten, a protein found in many grains such as wheat, barley, rye and oats)
2. Genetic susceptibility
3. Having an unusually permeable intestinal wall (a "leaky gut" as Dr. Fasano calls it)
What makes gluten so special? All proteins are made up of amino acids (20 or 22 of 'em, depending on who's counting), of which 8 are considered essential (meaning the body can't manufacture them, and must get them from the diet.) Most protein gets broken down quickly into amino acids, but gluten contains large amounts of glutamine and proline, 2 amino acids which resist easy breakdown. The resulting bigger chunks of protein cause an immune/inflammatory response (the gut makes a food/not food decision and says "not food"). The end result is slowly worsening absorption of vitamins, minerals and other nutrients. If you think of the intestines as a tube with fingers projecting from the inner wall, this immune response cuts the fingers off at the first knuckle, decreasing absorption. End result: abdominal pain, diarrhea, anemia, potentially death (up to 35% probability according to Dr. Fasano).
People of Celtic origin (Irish, Scottish, British, Welsh) have about a 5% likelihood of having the genetic susceptibility, but it can occur in others. Dr. Fasano quotes a 1 in 133 chance, meaning 45 million people may be at risk. Unfortunately, gluten may be giving us a double whammy - it may be the proximate cause of the leaky gut, also.
Speculation: Could gluten be the inciting cause (or at least the straw-camel-back) of all allergic disorders? Occam's Razor says at least "maybe." It would be useful to look at the rate of autoimmune diseases in southern China before the introduction of wheat into their diet (rice does not contain gluten) versus now.