Rather than a unique disorder, Crohn’s disease is regarded as a common denominator for various immune mediated inflammatory diseases of the gastrointestinal tract. These various subforms of Crohn’s disease are characterised by T cell mediated tissue destruction in the gut. In contrast with autoimmune diseases where autoantibodies react with self antigens, inflammation in Crohn’s disease is directed against antigens that are mainly derived from the environment.¹ Although it may sound trivial, this adjustment in the understanding of the pathogenetic mechanisms has a vast number of consequences for the search of the disease causing factors and identification of therapeutic targets.

CROHN’S PHENOTYPES

The various subforms of Crohn’s disease have not been recognised until recently. Historically, colonic Crohn’s disease was not distinguished from ulcerative colitis. Approximately 30 years ago, the first studies highlighted the diverse natural history of colonic and ileal Crohn’s disease.² Inherent differences in disease behaviour (obstructing versus perforating) were later recognised by a group of physicians at Mount Sinai Hospital in New York.³ Biological discrimination between various subgroups of Crohn’s patients led to a proposal for a phenotypic classification,⁴ which was taken up and further developed and evaluated by an international working group, leading to the Vienna classification.⁵ Accordingly, Crohn’s disease is separated into three categories: age at diagnosis, location, and behaviour (table 1). Several teams have now tested the Vienna classification in their databases^6–8 and found that it is feasible and reproducible. The behaviour and age categories earned some criticism which will be discussed later. The location category was found to be very useful and was also stable over time, with only one of seven patients having changes over a 10 year observation period.⁷

CLASSIFICATION BY AETIOLOGY

The best classification would be by cause of disease, but unfortunately this is …