Reactive arthritis is usually self‐limiting polyarthritis which develops in HLA‐B27 positive individuals after certain gastrointestinal or urogenital infections. The pathogenesis of reactive arthritis is unknown but T cells seem to have a crucial role. Most of the antigen‐specific T cells isolated from the synovial fluid have been MHC class II restricted. The role of antigen presentation in the pathogenesis of reactive arthritis has been studied relatively little. In this work the authors studied theeffect of arthritis‐triggering bacterium (Yersinia enterocoliticaO:3) on the expression of MHC class II molecules on human monocytes and found that the expression of different MHC class II molecules was regulated independently from each other in half of the individuals after certain incubation periods. In these cases the expression of HLA‐DP was parallel to the expression of HLA‐DQ, while HLA‐DR expression went to the opposite direction or did not change at all. No difference between HLA‐B27negative and HLA‐B27 positive healthy individuals was seen. The authors conclude that independent regulation of the expression of different MHC class II antigens on antigen‐presenting cells is a more common phenomenon t