Human leukocyte antigen (HLA) B27 (subtypes B*2701-2759) is a class I surface antigen encoded by the B locus in the major histocompatibility complex (MHC) on chromosome 6 and presents antigenic peptides (derived from self and non-self antigens) to T cells. HLA-B27 is strongly associated with ankylosing spondylitis (AS), and other associated inflammatory diseases referred to as "spondyloarthropathies". Diseases associated with the HLA-B27 subtype can be remembered with the mnemonic PAIR, and include Psoriasis, Ankylosing spondylitis, Inflammatory bowel disease, and Reiter syndrome.