Reactive arthritis: a paradigm for inflammatory arthritis.

Reactive arthritis (ReA) is an inflammatory arthritis which follows either chlamydia-induced non-specific urethritis or gastroenteritis due to yersinia, salmonella, shigella or campylobacter. It is distinguished from other infection-induced arthritides by its association with the MHC class I antigen HLA-B27, the pattern of arthritis (a lower-limb oligoarthritis often associated with sacroiliitis) and its systemic features (conjunctivitis, circinate balanitis and skin rash). ReA is unique among inflammatory arthritides in the clear definition of its trigger, its onset, its HLA association, and the demonstrating of a triggering antigen-specific cell-mediated immune response in the joint. Clear delineation of these factors makes it possible to test pathogenetic hypotheses which cannot be analysed in other more common forms of arthritis. However, since there are many similarities between these and ReA, the mechanisms established in ReA may have general relevance in understanding synovitis.