Gluten is a generic term for the storage proteins found in wheat, rye, and barley, but it is also found in all grains to some extent. Gliadin, the protein found in wheat, is responsible for the damage to the intestinal tract in celiac disease.
Science estimates that approximately 40 percent of the population reacts poorly to gluten, and the average onset of symptoms of gluten sensitivity is 35 to 45 years of age.
An allergy to gluten causes a direct immune system response. An intolerance has more to do with an inability to digest gluten. Undigested gluten will then contribute to abnormal growth of bacteria, the byproducts of which will cause an indirect reaction by the immune system.