page_title: MHC restriction
section_title: Structural specificity
hierarchical_section_title: MHC restriction / Introduction / Structural specificity
caption_attribution_description: English: Illustration of HLA-A (HLA A*0201-B2M complexed with HIV TAX peptide)from PDB:2git.
context_page_description: MHC-restricted antigen recognition, or MHC restriction, refers to the fact that a T cell can interact with a self-major histocompatibility complex molecule and a foreign peptide bound to it, but will only respond to the antigen when it is bound to a particular MHC molecule.
  When foreign proteins enter a cell, they are broken into smaller pieces called peptides. These peptides, also known as antigens, can derive from pathogens such as viruses or intracellular bacteria. Foreign peptides are brought to the surface of the cell and presented to T cells by proteins called the major histocompatibility complex. During T cell development, T cells go through a selection process in the thymus to ensure that the T cell receptor will not recognize MHC molecule presenting self-antigens, or rather it has a moderate affinity. High affinity means it will be autoreactive, but no affinity means it will not bind strongly enough to the MHC. The selection process results in developed T cells with specific TCRs that might only respond to certain MHC molecules but not others. The fact that the TCR will recognize only some MHC molecules but not others contributes to "MHC restriction".
context_section_description: The peptide-MHC complex presents a surface that looks like an altered self to the TCR. The surface consisting of two α helices from the MHC and a bound peptide sequence is projected away from the host cell to the T cells, whose TCRs are projected away from the T cells towards the host cells. In contrast with T cell receptors which recognize linear peptide epitopes, B cell receptors recognize a variety of conformational epitopes (including peptide, carbohydrate, lipid and DNA) with specific three-dimensional structures.
caption: HLA-A projected away from the cell surface and presenting a peptide sequence.
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