Crohn's Disease (CD) affects about 60.000 to 80.000 people in France. It is characterised by a chronic or relapsing inflammation of the gut. Its aetiology is largely unknown, limiting the development of preventive and curative therapeutic options. In 2001, we have identified the first CD susceptibility gene: CARD15/NOD2. This gene is involved in the innate immune response but we do not know today how gene mutations may induce the disease lesions. However, many data suggest that the intestinal inflammation may be related with an abnormal function of the lymphoid tissue present in the gut.