DefinitionHLA-B27 is a blood test to look for specific protein found on the surface of white blood cells. The protein is called human leukocyte antigen B27 (HLA-B27).Human leukocyte antigens (HLAs) are proteins that help the body's immune system tell the difference between its own cells and foreign, harmful substances.See also: Histocompatibility antigen testAlternative NamesHuman leukocyte antigen B27How the test is performedBlood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell with blood.A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band i