Most life forms have an innate immune system. Chrohn's Disease afflicts more than 500,000 people in the United States, most of whom are under 30. About one in four patients have a family history of the disease, which has encouraged researchers to find a genetic origin. How the mutation contributes to Crohn's Disease is unclear. So it's a very ancient part of the immune system.
Researchers believe that immune cells in the gut overreact to the presence of bacteria.
The U.S. study tested DNA supplied by 416 families with a history of Crohn's Disease. The French study tested 235 families.