During normal digestion, the body breaks down proteins into their smaller parts (amino acids) and these are absorbed across the wall of the intestine and delivered to the body via the blood. The body can then use them for all the good things they do like supporting growth and repair. If the gut is inflamed, as can be the case in some sensitive dogs, whole proteins can find their way across the wall of the small intestine and can arrive whole into the large intestine too. This is a big problem. These whole proteins can trigger an immune response (an allergic reaction). In other words, the dog can become sensitised to the type of proteins that have managed to seep through. This may have been chicken, beef etc. You may find that your dog reacts to certain foods with a skin flare up or loose stool. This shows us that the body is seeing them as an invader and has launched an immune response.