B27

B27 is most commonly used to refer to HLA-B27, a specific variant of the human leukocyte antigen B gene product. HLA-B27 is a protein that sits on the surface of many cells as part of the major histocompatibility complex class I system, where it helps present peptide fragments to immune cells. The HLA-B27 gene is inherited as part of the human genome, and the B27 antigen is evaluated through genetic or serologic testing in clinical settings.

The association with disease is a major reason for the prominence of HLA-B27. It is strongly linked to inflammatory conditions known as spondyloarthropathies, especially ankylosing spondylitis, and to related disorders such as reactive arthritis, psoriatic arthritis, and inflammatory bowel disease–associated arthritis. While the presence of HLA-B27 increases risk, it is not a definitive test for disease; many carriers remain healthy, and not all patients with these conditions carry the allele. The strength of the association varies by population and is influenced by additional genetic and environmental factors.

Testing and interpretation: HLA-B27 testing can aid in diagnosing suspected spondyloarthritis when clinical symptoms are present. Methods include serologic testing and high-resolution molecular typing. A positive result supports but does not confirm a diagnosis, while a negative result does not completely rule out disease in individuals with compatible symptoms.

Other uses: Outside immunogenetics, B27 can serve as a general alphanumeric designation in various contexts, such as cataloging, routing, or product codes, where its meaning is specific to the surrounding system.