<s>[INST] context: The function of MHC molecules is to display peptide fragments of protein antigens to antigen-specific T cells for recognition and activation of immune response. Because MHC molecules are fundamental to the recognition of antigens by T cells and are linked to many autoimmune diseases. In humans the MHC molecules are called human leukocyte antigens (HLA). On the basis of their structure, cellular distribution and function, MHC gene products are classified into two major classes: * Class I MHC molecules- expressed on all nucleated cells and platelets. They are heterodimers consisting of a polymorphic a, or heavy, chain linked non-covalently to a smaller non polymorphic protein called b 2 -microglobulin. Class I MHC molecules display peptides that are derived from proteins, such as viral and tumor antigens, that are located in the cytoplasm and usually produced in the cell, and class I-associated peptides are recognized by CD8+ T lymphocytes. Since CD8+ T cells recognize peptides only if presented as a complex with class I MHC molecules, CD8+ T cells are said to be class I MHC-restricted. * Class II MHC molecules- encoded in a region called HLA-D, which has three subregions: HLA-DP, HLA-DQ, and HLA-DR. Class II molecule is a heterodimer consisting of a noncovalently associated a chain and b chain, both of which are polymorphic. Class II MHC molecules present antigens that are internalized into vesicles, and are typically derived from extracellular microbes and soluble proteins. The class II b 2 domain has a binding site for CD4, and therefore, the class II-peptide complex is recognized by CD4+ T cells. Because CD4+ T cells can recognize antigens only in the context of self-class II molecules, they are referred to as class II MHC-restricted. The combination of HLA alleles in each individual is called the HLA haplotype. [/INST] question: The role played by major histocompatibility complex protein is: | options: A: Transduce the signals to T cells following antigen recognition B: Mediate immunoglobulin class switching C: Present antigens for recognition by T cell antigen receptor D: Enhance the secretion of cytokines | answer: C </s>