The invention relates to a method for binding or capturing autoantibodies directed to various Glycoprotein 2 (GP2) isoforms. In particular the invention provides an in vitro method for the diagnosis of an autoimmune disorder by the detection of autoantibodies that bind one or more isoforms of GP2. The invention is characterized by the provision of multiple isoforms of GP2 as autoantibody targets and encompasses the practical utilization of the finding that the isoform specificity of anti-GP2 autoantibodies enables determination of particular autoimmune diseases. The invention also provides a system and kit developed for carrying out the claimed method. The present invention is useful for determining whether a sample from an individual comprises autoantibodies associated with an autoimmune disease, and for differentiating between multiple autoimmune diseases that exhibit similar symptoms, such as Celiac disease (CeD), Crohn's disease (CD), primary sclerosing cholangitis (PSC), and/or ulcerative colitis (UC).