HLA-B27 (Human Leukocyte Antigen-B27) accounts approximately for the one third of the overall genetic susceptibility to spondylorthropathies (SpAs). Up to 70 HLA-B27 subtypes have been reported all over the world with a decreasing north-south gradient of its frequency, which is reverse to that of endemic malaria. In an attempt to explain the possible role of HLA-B27 in SpAs pathogenesis, several theories have been suggested [1. Arthritogenic peptide, 2. Misfolding, 3. Cell surface HLA-B27 homodimers, 4. β2m (β2-microglobulin) deposition, 5. β2m-free/peptide free heavy chains of HLA-I, 6. Enhanced survival of some microbes in HLA-B27 cells, 7. ERAP1/ERAP2 (endoplasmic reticulum aminopeptidases 1 and 2) and Tapasin function, 8. β2m over-expression] each of which contributes up to a point to our understanding of HLA-B27 subtypes' role in SpAs manifestation. However, reviewing all the suggested hypotheses it seems logical to pass from a puzzle of distinct hypotheses to a global theory. This review summarizes the current knowledge of HLA-B27 aiming to understand its potential use in clinical practice of SpAs diagnosis and to direct its future studies.
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