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- HLA-DQ (DQ) is a cell surface receptor protein found on antigen-presenting cells. It is an αβ heterodimer of type MHC class II. The α and β chains are encoded by two loci, HLA-DQA1 and HLA-DQB1, that are adjacent to each other on chromosome band 6p21.3. Both α-chain and β-chain vary greatly. A person often produces two α-chain and two β-chain variants and thus 4 isoforms of DQ. The DQ loci are in close genetic linkage to HLA-DR, and less closely linked to HLA-DP, HLA-A, HLA-B and HLA-C. Different isoforms of DQ can bind to and present different antigens to T-cells. In this process T-cells are stimulated to grow and can signal B-cells to produce antibodies. DQ functions in recognizing and presenting foreign antigens (proteins derived from potential pathogens). But DQ is also involved in recognizing common self-antigens and presenting those antigens to the immune system in order to develop tolerance from a very young age. When tolerance to self proteins is lost, DQ may become involved in autoimmune disease. Two autoimmune diseases in which HLA-DQ is involved are coeliac disease and type 1 diabetes. DQ mediates autoimmunity by skewing the T-cell receptor (TCR) repertoire during thymic selection. Carriers of risk serotypes such as DQ8 have a higher proportion of circulating T-cell receptors that may bind insulin, the primary autoantigen in type 1 diabetes. DQ is one of several antigens involved in rejection of organ transplants. As a variable cell surface receptor on immune cells, these D antigens, originally HL-A4 antigens, are involved in graft-versus-host disease when lymphoid tissues are transplanted between people. Serological studies of DQ recognized that antibodies to DQ bind primarily to the β-chain. The currently used serotypes are HLA-DQ2, -DQ3, -DQ4, -DQ5, -DQ6, -DQ7, -DQ8, -DQ9. HLA-DQ1 is a weak reaction to the α-chain and was replaced by DQ5 and DQ6 serology. Serotyping is capable of identifying most aspects of DQ isoform structure and function, however sequence specific PCR is now the preferred method of determining HLA-DQA1 and HLA-DQB1 alleles, as serotyping cannot resolve, often, the critical contribution of the DQ α-chain. This can be compensated for by examining DR serotypes as well as DQ serotypes. (en)
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