page_title: Major histocompatibility complex
section_title: Antigen processing and presentation
hierarchical_section_title: Major histocompatibility complex / Antigen processing and presentation
caption_attribution_description: English: Peptide binding for Class I and Class II MHC molecules, showing the binding of peptides between the alpha-helix walls, upon a beta-sheet base. The difference in binding positions is shown. Class I primarily makes contact with backbone residues at the Carboxy and amino terminal regions, while Class II primarily makes contacts along the length of the residue backbone. The precise location of binding residues is determined by the MHC allele.
context_page_description: The major histocompatibility complex is a large locus on vertebrate DNA containing a set of closely linked polymorphic genes that code for cell surface proteins essential for the adaptive immune system. This locus got its name because it was discovered in the study of tissue compatibility upon transplantation. Later studies revealed that tissue rejection due to incompatibility is an experimental artifact masking the real function of MHC molecules - binding an antigen derived from self-proteins or from pathogen and the antigen presentation on the cell surface for recognition by the appropriate T-cells. MHC molecules mediate interactions of leukocytes, also called white blood cells, which are immune cells, with other leukocytes or with body cells. The MHC determines compatibility of donors for organ transplant, as well as one's susceptibility to an autoimmune disease via cross-reacting immunization.
  In a cell, protein molecules of the host's own phenotype or of other biologic entities are continually synthesized and degraded. Each MHC molecule on the cell surface displays a small peptide, molecular fraction of a protein, called an epitope.
context_section_description: Peptides are processed and presented by two classical pathways:
  In MHC class II, phagocytes such as macrophages and immature dendritic cells take up entities by phagocytosis into phagosomes—though B cells exhibit the more general endocytosis into endosomes—which fuse with lysosomes whose acidic enzymes cleave the uptaken protein into many different peptides. Via physicochemical dynamics in molecular interaction with the particular MHC class II variants borne by the host, encoded in the host's genome, a particular peptide exhibits immunodominance and loads onto MHC class II molecules. These are trafficked to and externalized on the cell surface.
  In MHC class I, any nucleated cell normally presents cytosolic peptides, mostly self peptides derived from protein turnover and defective ribosomal products. During viral infection, intracellular microorganism infection, or cancerous transformation, such proteins degraded in the proteosome are as well loaded onto MHC class I molecules and displayed on the cell surface. T lymphocytes can detect a peptide displayed at 0.1%-1% of the MHC molecules.
caption: Peptide binding for Class I and Class II MHC molecules, showing the binding of peptides between the alpha-helix walls, upon a beta-sheet base. The difference in binding positions is shown. Class I primarily  makes contact with backbone residues at the Carboxy and amino terminal regions, while Class II primarily makes contacts along the length of the residue backbone. The precise location of binding residues is determined by the MHC allele.[20]
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