HLA-B27: A Key Genetic Marker in Ankylosing Spondylitis and Related Disorders

The human leukocyte antigen HLA-B27 is a variant of the class I major histocompatibility complex (MHC I) molecule. It is encoded by the HLA-B locus on chromosome 6 and expressed on almost all nucleated cells.

Role in Health and Disease

HLA-B27’s normal function is to present peptide antigens (derived from both self and non-self proteins) to CD8 + T-cells, helping the immune system respond to infections and maintain immune surveillance.

However, HLA-B27 is also strongly associated with inflammatory disorders — most notably Ankylosing Spondylitis (AS), a type of axial spondyloarthritis that affects the spine and sacroiliac joints.

Key Association Facts

• Among AS patients, HLA-B27 positivity ranges broadly from about 60% to 90% globally (depending on region and ethnicity).
• In the general population (without disease), HLA-B27 positivity varies by ethnicity — for example, about 8% in many European populations.
• Having HLA-B27 does not guarantee disease. Many carriers never develop AS or related conditions.

Why HLA-B27 Matters for Testing

Detection of HLA-B27 is clinically valuable as part of the diagnostic work-up for suspected spondyloarthritis, especially in patients with early onset back pain, sacroiliac involvement, or extra-articular features such as anterior uveitis.
Because HLA-B27 is strongly associated with AS and other “seronegative” arthritides (e.g., reactive arthritis, psoriatic arthritis) it helps stratify risk and can support earlier diagnosis.

Allelic Diversity of HLA-B27

HLA-B27 is not one single sequence — there are many alleles/sub-types (for example B2701, B2702, B*2705, etc.).
While many of these alleles are associated with AS risk, some are less strongly associated or even appear protective. For example:

• The allele HLA‑B27:06 (common in Asia) and HLA‑B27:09 (found in Sardinia) appear to have minimal or no increased AS-risk despite being HLA-B27 positive.
• Because of this allelic variation, subtyping or high-resolution typing of HLA-B27 can provide additional insight beyond simple positivity.

Radova Haluza