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Human leukocyte antigen (HLA) is the gene expression product of the major histocompatibility complex (MHC), which was first discovered on leukocytes and platelets, but widely exists on nuclear envelopes in many tissues. HLA can be divided into class I molecules, class II molecules and class III molecules according to the antigen structure. Class I molecules include HLA-A, HLA-B, HLA-C series antigens and some other pseudogenes such as HLA-H, HLA-J, HLA-K, etc. while class II molecules include HLA-D/DR, HLA-DP, HLA-DQ series. Class III molecules are complement components. HLA-b27 is a typical MHC class I molecule, composed of a cell membrane glycoprotein A chain (molecular weight 44KDa, heavy chain) and an extracellular β2 microglobulin. The heavy chain can be divided into extracellular active region, transmembrane region and cytoplasmic region. The extracellular area consists of α1, α2, α3 active area, and an antigen polypeptide binding pocket is formed between α1 and α2, where proteolytic polypeptides can stick in and be recognized by immune cells such as CD8+T cells. When HLA-B27 presenting its own peptide fragments, it will lead to the attack of the autoimmune system, thus causing ankylosing spondylitis (AS). There is a strong correlation between the expression abundance of HLA-B27 and AS, and the positiveexpression rate of HLA-B27 in patients with AS is up to 90%. So the test results of HLA-B27 can assist the clinical diagnosis of AS, but the positive or negative results of the HLA-B27 detection cannot be used as the basis for diagnosing or excluding AS.

References

[1] Fiorillo M T, Maragno M , ButlerR, etal. CD8+ T-cell autoreactivity to an HL A-B27-restricted self-epitope correlates with ankylosing spondylitis[J]. Journal of Clinical Investigation, 2000, 106(1).