There are foods that the immune system recognizes as gluten in addition to wheat, rye, and barely. This process is called cross-reactivity
A few examples of such foods include, casein ( milk protein)
corn, yeast, millet, rice, sesame, soy
Some hidden sources of gluten include modified food starch, food emulsifiers, food stabilizers, artificial coloring, processed ketchup, mustard, certain salad dressings, deli meats, beer, soy sauce, and shampoos.