HLA-DQ is also known as MHC class II DQ monomorphic antigen. The major histocompatibility complex is composed of two heterodimeric glycoproteins (alpha and beta chains) with apparent molecular weights of 27 and 32 kD. In contrast to other MHC class II molecules, both polypeptide chains of HLA-DQ are polymorphic, with the alpha chain showing an extremely high degree of polymorphism. HLA-DQ is expressed on B cells in the peripheral blood, and weakly expressed on activated T cells and some monocytes. HLA-DQ is absent on hematopoietic progenitors, resting T cells, erythrocytes, and platelets. HLA-DQ is expressed after HLA-DR and HLA-DP in hematopoietic development. HLA-DQ presents peptide fragments mainly from degraded intravesicular and extracellular proteins to CD4+ T lymphocytes. Specific alleles of HLA-DQ have been linked to the pathogenesis of several autoimmune diseases (including diabetes), both as a susceptibility and resistance factor depending on the particular polymorphism.