HLA-DQ is also known as MHC class II DQ monomorphic antigen. The major histocompatibility complex is composed of two heterodimeric glycoproteins (α and β 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 α 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.