Clone REA303 recognizes HLA-DQ, an αβ heterodimer of the MHC class II type. Expressed on the surface of most nucleated cells, MHC class II molecules play an important role in the immune system. They are essential in the defense against infection and are a main consideration in transplantation medicine. In addition to presenting antigenic peptides from predominantly extracellular sources to CD4

 T cells, MHC class II molecules also mediate the thymic selection of T helper cells. MHC class II molecules consist of an α and β chain and are transported to endosomal-lysosomal compartments by the invariant chain. In humans, MHC class II molecules are encoded by three different loci, HLA-DR, -DQ, and -DP, which display 70% similarity to each other. Both chains of HLA-DQ are polymorphic. Despite the essential function of MHC class II molecules in immune defense against pathogens, some alleles are frequently linked to immune diseases. Two autoimmune diseases in which HLA-DQ is involved are coeliac disease and diabetes mellitus type 1.