Human Leukocyte Antigen B27 (subtypes B*2701-2724) is a class I surface antigen encoded by the B locus in the major histocompatibility complex (MHC) on chromosome 6 and presents microbial antigens to T-cells. HLA-B27 is strongly associated with a certain set of autoimmune diseases referred to as the "seronegative spondyloarthropathies".

One form is ankylosing spondylitis which is a chronic, inflammatory arthritis. It affects joints in the spine and the sacroilium in the pelvis, causing eventual fusion of the spine.  Early onset occurs from 18-30 years of age.  In the general population, about 8% Caucasian, 4% African, 2-9% Chinese, and 0.1-0.5% Japanese have the HLA-B27 antigen.