Study of products of genes present on a region on chromosome 6 known as the Major Histocompatibility Complex (MHC) is providing new insight into many chronic diseases of undetermined aetiology, including rheumatic diseases such as ankylosing spondylitis1,2 and rheumatoid arthritis3,4. The human MHC is called HLA, and it contains a tightly linked cluster of genes that encode for cell surface glycoprotein molecules expressed on the cell membrane of virtually all cells5–7. These molecules are involved in cell-to-cell interaction and have been subdivided into two distinct groups called class I and class II molecules6–8. The class I molecules are encoded by the HLA-A, -B, and -C loci of the MHC. These molecules display an exceptional degree of genetic polymorphism and are composed of an MHC-encoded heavy — or α — chain that is non-covalently bound to β2-microglobulin, a smaller and invariant polypeptide chain encoded by a gene located on another chromosome5–8. The class II molecules consist of two glycoprotein chains as well, the larger of the two chains is called α chain and the smaller one is called β chain. These two chains are closer in size than those of the class I molecules. Moreover, both α and β chains are encoded by genes located in the HLA-D region7,8.