Human leukocyte antigen B27 (HLA-B27) is a protein located on the surface of your white blood cells. An HLA-B27 test is a blood test that identifies HLA-B27 proteins.
Human leukocyte antigens (HLAs) are proteins commonly found on white blood cells. These antigens help your immune system identify the differences between healthy body tissue and foreign substances that may cause infection.
Although most HLAs protect your body from harm, HLA-B27 is a specific type of protein that contributes to immune system dysfunction. The presence of HLA-B27 on your white blood cells can cause your immune system to attack those otherwise healthy cells. When this occurs, it can result in an autoimmune disease or immune-mediated disease, such as juvenile rheumatoid arthritis or ankylosing spondylitis.