Dixie made a lot of great points, most of which I agree with.
However, the overdrive theory was popular for a very long time, but now researchers are starting to look in another direction. In fact, within the past two years evidence has suggested that Crohn's patients have under-active immune systems, not overactive ones.
Why is why there are clinical trials going on right now that use meds that do NOT suppress the immune system.
There is no question that Crohn's patients have dysfunctional immune systems. That something in the environment sets off a trigger and then inflammation sets in. However, it is possible that instead of having overactive systems, they have weak or at least inefficient systems. That an attack is mounted that might resolve itself easily in somebody who does not have Crohn's. But in a Crohn's patient, the attack is inneffectual...not strong enough to finish the job so instead you get stuck in the fester mode.
The immunosuppresants do a good job of suppressing the damage that can be done by Crohn's. However, they are not just suppressing this action, but the immune system for your entire body. A small minority of researchers have even speculated that the suppression of the Crohn's inflammation might not be good over the long run, because it does not really get to the root of what caused the inflammation, to begin with.
I don't believe in the overdrive theory, because my daughter's immune system was terrible long before she had Crohn's.