I found this on the CCFA website:
"The immune system is composed of various cells and proteins. Normally, these protect the body from infection. In people with Crohn's disease, however, the immune system reacts inappropriately. Researchers believe that the immune system mistakes microbes, such as bacteria that is normally found in the intestines, for foreign or invading substances, and launches an attack. In the process, the body sends white blood cells into the lining of the intestines, where they produce chronic inflammation. These cells then generate harmful products that ultimately lead to ulcerations and bowel injury. When this happens, the patient experiences the symptoms of IBD.
Studies indicate that the inflammation in IBD involves a complex interaction of factors: the genes the person has inherited, the immune system, and something in the environment. Foreign substances (antigens) in the environment may be the direct cause of the inflammation, or they may stimulate the body's defenses to produce an inflammation that continues without control. Researchers believe that once the IBD patient's immune system is "turned on," it does not know how to properly "turn off" at the right time. As a result, inflammation damages the intestine and causes the symptoms of IBD. That is why the main goal of medical therapy is to help patients regulate their immune system better."
It is degenerative in that eventually it leads to severe damage to the areas being attacked by the immune system;especially if it doesn't respond well to the medication.
CCFA has alot of good educational information about Crohn's disease and ulcerative colitis; also there are many people here that are WAY better educated than me on this disease lol. Just search the threads.