HLA-DRB1 and rheumatoid arthritis: In fact, the abundance of activated CD4+ T cells in the synovium of individuals with RA, especially people who express certain allelic forms of the MHC II β-chain (e.g., HLA-DRB1*0401), indicates that the process of antigen recognition by CD4+ cells plays a critical role in the pathogenesis of RA [1].