h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

It is interesting that you say that the cause of Crohn's is unknown. There has been a lot of recent work on Crohn's Disease [CD] (and its cousin disease, Ulcerative Colitis [UC]) that have revealed some interesting features.

It appears that CD/UC is the outcome response of the necessary interplay between the following three players:

1. tissue type (the so called 'HLA antigens', which is a misnomer as HLA stands for human lymphocyte antigen - so I am actually saying human lymphocyte antigen antigens which is BAD ENGLISH and as bad as getting money from an ATM machine!),
2. the bacteria in the gut (apparently the resident populations have a lot to do with your mother and the dirt that you eat as a child! SO if you have a Drop-Dead-Fred kind of mother (cobwebs in her undies) and don't eat much dirt, you're behind the 8 ball) and
3. the usual immune responses when the bacteria gain entry to the gut lining (in this regard there appears to be substantial tolerance by our immune systems to the normal bacteria, almost as if the immune system turns a blind eye to the bacteria it expects to see).

Apparently the HLA type affects the bacterial population. The unusual bacterial population that results will eventually enter the gut wall (this sort of breach happens all the time in the intestine) to produce one of two major immune responses. One response majors on cells that produce scarring and so looks like CD, while the other majors on cells of acute inflammation and so looks like UC.

So CD and UC are really diseases of disordered immunity, although the 'disorder' is the result of tissue type and subsequent bacterial population dynamics. This is why the disease has always been labelled as having a mixture of causes.

The use of immune cell communicator molecules such as IL10 (interleukin #10 - this is bad news for science students as they will very quickly realise that there are ANOTHER nine intereukins to learn about; and even worse news is that IL12 is also involved!) can remove the disease manifestations. There are now some normal harmless bacteria that have been engineered to produce the IL10 with good therapeutic effects in mice (which is very useful if you are a mouse, but has not reached humans yet). So the answer to this disease which is partly caused by bad bacteria, may be to eat a good bacteria supplement!

I learnt all this very recently, and was absolutely fascinated. Here is a disease caused because the body's cellular identification tags interact with a bacteria's identification tags. And justifies why I should keep my children dirty!