HLA-DRB1 and rheumatoid arthritis: The HLA-DRB1 allele associated with MHC is the most popular genetic factor in RA, and it increases the risk (HLA-DRB1*0401, *0404/*0408, *0405, *0101, *1001, and *1402), whereas HLA-DRB1*13 has a protective effect and decreases the risk [3,20,23,24,25,26].