Among the strongest associations between HLA haplotype and rheumatic disease is the correlation between expression of the MHC Class I allele HLA-B27 and the incidence of spondyloarthropathy. The spondyloarthropathies are a group of related diseases typified by sacroiliitis, peripheral arthritis, and enthesitis, or inflammation at the site of the attachment of skeletal muscle, tendons, ligaments, or joint capsule to bone (for a review of the spondyloarthropathies see Kataria et al., 2004) . Prominent among the spondyloarthritides is ankylosing spondylitis (AS), a chronic disease involving progressive ossification of the vertebrae and a host of other symptoms, including arthritis in the hips and shoulders, enthesitis, and acute uveitis. AS affects as much as 0.5% of the population with 96% of affected individuals express ing HLA-B27 . A second spondyloarthropathy showing high correlation with HLA-B27 expression is reactive arthritis (ReA), an acute disease that is precipitated by infection in the genitourinary or gastrointestinal tracts and is typified by joint inflammation, enthesitis, and uveitis. ReA affects only 0.1% of the population yet 80% of reactive arthritis patients express HLA-B27.
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