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The Major Histocompatibility Complex (MHC) encodes a family of proteins that are central for the activation of an immune response. The class I and class II MHC molecules are integral membrane proteins expressed at the cell surface. They bind peptide fragments of antigen and present them to T cells that express a complimentary T cell receptor for antigen. In order for T lymphocytes to be activated and respond to antigenic stimuli, their T cell receptor for antigen must see antigen complexed to an MHC class I or class II molecule. MHC class I molecules are expressed on virtually all nucleated cells and present endogenously synthesized antigens to CD8+ T lymphocytes. In contrast, MHC class II molecules are only expressed on so-called “professional” antigen presenting cells, such as dendritic cells, macrophages, and B lymphocytes, and present antigens that are synthesized exogenously and imported into the antigen presenting cell by endocytosis. Development of effective T cell-mediated immunity usually requires activation of both CD4+ and CD8+ T cells, and hence MHC class I and II molecules are critical elements for the generation of an effective immune response.
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