Abstract
Inflammatory bowel diseases (IBD) are probably multifactorial diseases caused by an interplay of the external and internal environment. External environmental factors revealed in the literature so far can be grouped into four major groups or hypotheses: (a) the sheltered child, (b) infectious agents, (c) dietary factors, and (d) vascular factors. Internal environmental factors mainly concern genetics which have been studied in families or twins. At present, HLA class II antigens are most in focus but also genetic markers such as pANCA. However, the internal environment can be changed by external factors changing the balance of the faecal or mucosa related flora, such as infectious agents or antibiotic treatment. This in turn may upset the building up and degradation of colonic mucin, making the mucosa more vulnerable. The increasing knowledge of external and internal environmental factors might affect our way of treatment in the future. So also may our increasing knowledge of immunology which will probably lead to targeted treatment directed against individual components within the inflammatory cascade.