If you have this, your body reacts to gluten, a protein found in wheat and other grains. When you eat it, your immune system -- which helps you fight off bacteria and other germs -- attacks your small intestine by mistake. That can make it hard for your body to take in nutrients, and that can lead to weight loss. You also may have headaches, itchy skin, sores in your mouth, and joint pain. Only a doctor can tell you for sure if you have it. If you do, you’ll need to follow a special diet to stop the symptoms.