HLA-B27 is a specific protein found on the surface of cells that plays a crucial role in the immune system. It is part of a group of molecules known as human leukocyte antigens (HLA), which help the body recognize foreign substances. The presence of HLA-B27 is associated with certain autoimmune diseases, particularly ankylosing spondylitis and other spondyloarthritis conditions. Not everyone with HLA-B27 will develop these diseases, but the presence of this antigen can increase the risk. Testing for HLA-B27 is often done to help diagnose these conditions, especially in individuals with symptoms like chronic back pain or joint inflammation.