Introduction Contents: General Introduction Immunogenetics: host genetics and the immune system Candidate gene approach Genetic variation of the immune system and its mediators Chlamydia trachomatis infections of the urogenital tract Inflammatory Bowel Diseases Arthralgia and spondyloarthropathy Thromboembolism

General Introduction Immunogenetics: host genetics and the immune system Candidate gene approach Genetic variation of the immune system and its mediators Chlamydia trachomatis infections of the urogenital tract Inflammatory Bowel Diseases Arthralgia and spondyloarthropathy Thromboembolism Immunomodulatory aspects of probiotics The immune response and genetic predisposition have been implicated in the pathogenesis of several complex diseases in which the identification of genetic determinants constitutes a major challenge. Infectious diseases such as those associated with Chlamydia trachomatis (Ct) infection and chronic inflammatory bowel diseases are complex diseases in which multiple genetic, environmental, and pathogenic factors are likely to contribute to pathogenesis. The inflammatory bowel diseases Crohn's disease and ulcerative colitis are complex and multifactorial diseases of which the exact aetiology is not yet known. Crohn's disease may affect any part of the gastrointestinal tract from the oral cavity to the rectum, whereas ulcerative colitis is a disease of the large bowel only. Both are chronic diseases where environmental, immune-mediated and genetic factors determine the course. Several studies have failed to show the involvement of a specific bacterium in the aetiology of these diseases. The disease course might be asymptomatic at presentation or life-threatening. Patients may require lifelong immune suppressive treatment and in most cases surgical intervention. Often recurrences of intestinal inflammation have a severe impact on the patient's quality of life. Extraintestinal manifestations of these diseases also play an important role in the quality of life and morbidity. Chlamydia trachomatis is an obligate intracellular bacterium causing most cases of sexually transmitted infection with subsequent inflammation resulting in reproductive tract destruction and infertility. The exact aetiology needs to be elucidated. A common factor between the gastrointestinal and urogenital tract is the mucosal surface that is continuously exposed to potential pathogens as well as beneficial commensal microorganisms, and forms the first station for the immune system. Both tracts are part of the mucosal immune system. The small intestine plays a dual role which consists of exclusion of pathogens while facilitating absorption of nutrients. Homeostasis and balance between immunity and tolerance challenges the immune system of the mucosa. Remarkable interindividual variations have been observed in susceptibility to infection and inflammation, in the course of eradication of pathogens, and the development of complications. These variations might be due to differences in the genetic predisposition with regard to the immune regulation. The advances in human genetics and in particular in relation to the control of the immune …