There are at least three common features among the microbes causing reactive arthritis: (1) they primarily cause infections on mucosal areas, (2) they are intracellularly living microorganisms, and (3) they have lipopolysaccharide as an essential structure of their outer membrane. It is obvious that during or after the acute infection microbial antigens are not properly eliminated, and they persist for long times in HLA-B27-positive persons developing ReA. Bacterial lipopolysaccharide has also been shown to enter the joints and is an important factor in the pathogenesis of ReA.