HLA-DRB1 and juvenile idiopathic arthritis: Although RF + pJIA may be considered the paediatric counterpart of adult RA, with a quite similar genetic background (such as the susceptibility for HLA-DRB1 and many clinical features), several differences have been highlighted with the other JIA subtypes, including the poor involvement of RF and ACPA in JIA pathogenesis, the centrality of CD4+ Th17/Th1 cells in oJIA, the importance of HLA-B27 in ErA and the autoinflammatory characteristics of sJIA.