The role played by major histocompatibility complex 1 and 2
Correct Answer: Present antigens for recognition by T cells antigen receptor
Description: Ref Robbins 8/e p191 Class I MHC molecules are encoded by three closely linked loci, designated HLA-A, HLA-B, and HLA-C (Fig. 4-3). Each of these molecules is a heterodimer, con- sisting of a polymorphic 44-kDa a chain noncovalently associated with an invariant 12-kDa b2-microglobulin polypeptide (encoded by a separate gene on chromo- some 15). The extracellular poion of the a chain contains a cleft where the polymorphic residues are located and where foreign peptides bind to MHC mol- ecules for presentation to T cells, and a conserved region that binds CD8, ensuring that only CD8+ T cells can respond to peptides displayed by class I molecules. In general, class I MHC molecules bind and display pep- tides derived from proteins synthesized in the cyto- plasm of the cell (e.g., viral antigens). Because class I MHC molecules are present on all nucleated cells, all virus-infected cells can be detected and eliminated by CD8+ CTLs. Class II MHC molecules are encoded by genes in the HLA-D region, which contains at least three subregions: DP, DQ, and DR. Class II MHC molecules are heterodi- mers of noncovalently linked polymorphic a and b sub- units (Fig. 4-3). The extracellular poion of the class II MHC heterodimer contains a cleft for the binding of antigenic peptides and a region that binds CD4. Class II MHC expression is restricted to a few types of cells, mainly APCs (notably, dendritic cells ), macro- phages, and B cells. In general, class II MHC molecules bind to peptides derived from proteins synthesized outside the cell (e.g., those derived from extracellular bacteria) and ingested into the cell. This propey allows CD4+ T cells to recognize the presence of extracellular pathogens and to orchestrate a protective response. * Several other proteins are encoded in the MHC locus, some of which have been called "class III molecules." These include complement components (C2, C3, and Bf) and the cytokines tumor necrosis factor (TNF) and lymphotoxin. These molecules do not form a pa of the peptide display system and are not discussed fuher.
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