HLA B27

About HLA B27

HLA B27 is an allele group of the HLA B gene, which encodes an MHC class I molecule that presents peptides to CD8 T cells, located in the major histocompatibility complex on chromosome 6p21.33. It is carried by a small percentage of people of European ancestry, with frequency varying widely between populations and highest in some Indigenous peoples of the Arctic. HLA B27 is present in the large majority of people with ankylosing spondylitis, one of the strongest associations between an HLA type and a disease, first reported in 1973. It is also associated with reactive arthritis, acute anterior uveitis, and spondyloarthritis linked to psoriasis and inflammatory bowel disease. However, only a small minority of HLA B27 carriers develop ankylosing spondylitis, so other genes and environmental factors are involved. Proposed mechanisms include presentation of arthritogenic peptides, misfolding of the HLA B27 heavy chain causing endoplasmic reticulum stress and inflammation, formation of abnormal heavy chain dimers, and molecular mimicry with bacteria such as Klebsiella. HLA B27 testing supports the diagnosis of spondyloarthritis but is not diagnostic alone. Many subtypes exist, and some, such as B2706 in Southeast Asia and B2709 in Sardinia, appear to be weakly or not associated with disease.