Gluten is combination of specific proteins that is only found in the seeds of wheat, rye, barley and spelt. Other grains, such as corn and rice, contain different proteins to serve the same functions.
Gluten consists of the proteins gliadin and glutenin, which bind to starch in wheat and related seeds. The substance nourishes the seeds as they start to grow. Harvested seeds from wheat retain the gluten, and it remains in flour and other processed products made from the seeds. Corn, oats, rice and millet are not close relatives of wheat and do not have the gluten protein combination inside their seeds. Occasionally, products made of these grains become contaminated with gluten during processing, but the actual plants and seeds are always gluten-free in their natural state.
Gluten is different from other proteins found in seeds because of its pliability. This makes gluten-rich flours ideal for using in yeast breads. As the yeast produces gas, the gluten stretches, allowing the bread dough to rise and creating a texture with air pockets.
For some people, gluten causes health issues such as gastrointestinal distress and a decrease in the ability to absorb nutrients from food. Celiac disease is a severe form of gluten sensitivity.