HLA-B27 (Human Leukocyte Antigen B27) is a specific genetic variant of the HLA-B gene located on chromosome 6. While HLA proteins normally help the immune system distinguish between the body’s own cells and foreign invaders, the B27 variant is a well-known risk factor for a group of autoimmune disorders known as seronegative spondyloarthropathies.
Principle
The test operates on the principle of detecting either the HLA-B27 protein on the surface of lymphocytes or the HLA-B27 gene in the individual’s DNA.
Protein Detection: Uses specific antibodies that bind to the HLA-B27 antigen if it is present on the white blood cell surface.
Genetic Detection: Uses molecular techniques to identify the specific DNA sequence corresponding to the HLA-B27 allele.
Procedure
Testing typically involves a standard blood draw (venipuncture). Common laboratory methods include:
Flow Cytometry: A rapid, high-throughput method where blood cells are tagged with fluorescent antibodies and passed through a laser to detect the B27 protein.
PCR (Polymerase Chain Reaction): The current “gold standard” molecular method that amplifies and identifies the HLA-B27 gene directly from leukocyte DNA.
Microlymphocytotoxicity (MLCT): A traditional serological method where lymphocytes are incubated with B27-specific antisera and complement; cell death indicates a positive result.
Result Interpretation
Results are generally reported as Positive or Negative.
Positive Result: Indicates the presence of the HLA-B27 antigen/gene. This signifies a higher statistical risk for associated autoimmune disorders, but is not a diagnosis on its own.
Fig. Interpretation of HLA-B27 Result
Negative Result: Indicates the antigen was not detected. While it significantly reduces the likelihood of HLA-B27-related diseases, it does not completely rule them out, as some patients develop these conditions without the marker.
Clinical Significance
The test is primarily used to support a diagnosis of:
Ankylosing Spondylitis (AS): Strongest association; ~90% of AS patients are HLA-B27 positive.
Reactive Arthritis (Reiter’s Syndrome): Often triggered by an infection.
Acute Anterior Uveitis: Recurring inflammation of the eye.
Psoriatic Arthritis & Inflammatory Bowel Disease (IBD)-associated Arthritis: Other conditions within the seronegative spondyloarthropathy family.
Keynotes
Not Diagnostic Alone: Most people with the HLA-B27 gene never develop an autoimmune disease.
Demographics: Prevalence varies by ethnicity; for example, it is more common in Northern European populations and less common in African American populations.
Lifelong Marker: Once a person tests positive, they will always be positive; the test does not need to be repeated.
Cross-Reactivity: Some tests (like flow cytometry) may occasionally show false positives due to cross-reactivity with similar antigens like HLA-B7.