Why are patients with IBD more prone to develop an inflammatory arthritis

Why are patients with IBD more prone to develop an inflammatory arthritis? 

• The pathogenesis of gut-joint iteropathy is unknown. However, inflammation of the gut and joints appear to be tightly linked.
• When ileocolonoscopies are done on spondyloarthropathy (ankylosing spondylitis, reactive arthritis) patients without GI symptoms, up to 25% have macroscopic lesions and up to 60% have microscopic evidence of asymptomatic CD.
• Over time, 6% to 10% of these patients develop overt symptomatic CD. Alternatively, up to 10% of IBD patients without evidence of a spondyloarthropathy at onset of their GI symptoms will develop overt arthritis on followup. 
• Environmental antigens capable of inciting rheumatic disorders enter the body’s circulation by traversing the respiratory mucosa, skin, or GI mucosa.
• The human GI tract has an estimated surface area of 400 m ² (200 times the body’s skin surface area) and functions not only to absorb nutrients but also to exclude potentially harmful antigens.
• The gut-associated lymphoid tissue, which includes Peyer patches, the lamina propria, and intraepithelial T cells, constitutes 25% of the GI mucosa and helps to exclude entry of bacteria and other foreign antigens.
• Whereas the upper GI tract is normally exposed to 10 ³ mucosa-adhering bacteria, the lower GI tract is constantly in contact with millions of bacteria (up to 10 ¹² /g of feces).
• The total number of bacterial cells called the human microbiota that we are exposed to is 10 times that of the number of cells of the body. 
• Inflammation, whether from idiopathic IBD or from infection with pathogenic microorganisms, can disrupt the normal integrity and function of the bowel, leading to increased gut permeability.
• This increased permeability may allow nonviable bacterial antigens in the gut lumen to enter the circulation more easily.
• These microbial antigens could either deposit directly in the joint synovia, leading to a local inflammatory reaction, or cause a systemic immune response, resulting in immune complexes that then deposit in joints and other tissues.
• Genetic susceptibility is required to develop the immunologic response in the gut and joint, which results in persistent inflammation and tissue injury.