HLA B27

Function

The HLA-B27 (Human Leukocyte Antigen B27) is a specific protein, known as a class I surface antigen, found on the surface of white blood cells. It is part of the Major Histocompatibility Complex (MHC), which plays a pivotal role in the immune system's ability to distinguish between its own cells and foreign invaders like bacteria or viruses. While the presence of HLA-B27 is normal in a small percentage of the healthy population, its existence is strongly linked to several autoimmune and inflammatory conditions, particularly those involving the spine and joints.

Why it is Ordered

Physicians order the HLA-B27 test when a patient exhibits symptoms of chronic inflammatory arthritis, such as persistent back pain, joint stiffness, or inflammation of the eyes (uveitis). It is not a diagnostic test on its own but serves as a critical piece of the puzzle when Suspecting Seronegative Spondyloarthropathies. It helps differentiate these conditions from other forms of arthritis like Rheumatoid Arthritis.

Associated Conditions

The presence of the HLA-B27 antigen is most notably associated with:

• Ankylosing Spondylitis (AS): A form of arthritis that primarily affects the spine, leading to fusion of the vertebrae.
• Reactive Arthritis: Joint inflammation triggered by an infection in another part of the body (formerly known as Reiter's Syndrome).
• Psoriatic Arthritis: Inflammation associated with the skin condition psoriasis.
• Acute Anterior Uveitis: Sudden inflammation of the middle layer of the eye.
• Juvenile Idiopathic Arthritis: Inflammation in children that can mimic adult spondyloarthropathies.