Two methods exist for an antigen to be processed and presented on the cell surface. Both MHC class I and II require a peptide to be bound for them to be stable. Otherwise, they are not expressed on the cell surface. An antigen is a molecule that stimulates the production of antibodies. ... MHC may be: an abbreviation for Managed health care an abbreviation for major histocompatibility complex the US Navy hull classification symbol for Coastal Minehunter the Mile High Club This page concerning a three letter acronym is a disambiguation page — a navigational aid which lists other pages that might otherwise share...

The Endogenous Pathway

The endogenous pathway is used to present cellular peptide fragments on the cell surface on MHC class I molecules. If a virus had infected the cell, viral peptides would also be presented, allowing the immune system to recognize and kill the infected cell. Worn out proteins within the cell become ubiquitinated, marking them for proteasome degredation. Proteasomes break the protein up into peptides that include some around nine amino acids long (suitable for fitting within the peptide binding cleft of MHC class I molecules). TAP, a protein that spans the membrane of the rough endoplasmic reticulum, transports the peptides into the lumen of the rough endoplasmic reticulum. Also within the rough ER, a series of chaperone proteins, including calnexin, calreticulin, ERp57, and immunoglobulin binding protein (BiP) facilitates the proper folding of class I MHC and its association with ß2 microglobulin. The partially folded MHC class I molecule then interacts with TAP via tapasin (the complete complex also contains calreticulin and Erp57 and, in mice, calnexin). Once the peptide is transported into theER lumen it binds to the cleft of the awaiting MHC class I molecule, stabilizing the MHC and allowing it to be transported to the cell surface by the golgi apparatus. Peptides (from the Greek πεπτος, digestable), are the family of molecules formed from the linking, in a defined order, of various amino acids. ... Ubiquitin es un peque�o prote�na que ocurre en la mayor�a eukaryote|eukaryotic c�lula biolog�acells. ... A proteasome is a barrel-shaped multi-protein complex that can digest other proteins into short polypeptides and amino acids in an ATP-driven reaction. ... The endoplasmic reticulum or ER (endoplasmic means within the cytoplasm, reticulum means little net) is an organelle found in all eukaryotic cells. ... A small protein that forms part of the Class I MHC (Major Histocompatibility Complex). ... In cell biology, the Golgi apparatus, Golgi body, Golgi complex, or dictyosome is an organelle found in most eukaryotic cells, including those of plants, animals, and fungi. ...

The Exogenous Pathway

The exogenous pathway is utlized by professional antigen presenting cells to present peptides derived from proteins that the cell has endocytosed. The peptides are presented on MHC class II molecules. Proteins are endocytosed are degraded by acid-dependent proteases in endosomes. The nascent MHC class II protein in the rough ER has its peptide binding cleft blocked by Ii (the invariant chain; a trimer) to prevent it from binding peptides cellular peptides or those from the endogenous pathway. The invariant chain also facilitates MHC class II's export from the ER in a vesicle. This fuses with a late endosome containing the endocytosed, degraded proteins. Ii is then broken down in stages, leaving only a small fragment called CLIP which still blocks the peptide binding cleft. An MHC class II-like structure, HLA-DM, removes CLIP and replaces it with a peptide from the endosome. The stable MHC class-II is then presented on the cell surface. An antigen presenting cell (APC) is a cell that displays foreign antigen complexed with MHC on its surface. ... In biology an endosome is an endocytotic vesicle derived from the plasma membrane. ...

There is also a mechanism to associate peptides from endocytosed proteins with MHC class I, termed cross-presentation. MHC class I molecules are cell surface proteins found on most cells of the body. ... The term cross-presentation denotes the ability of certain antigen-presenting cells to take up, process and present extracellular antigens with MHC class I molecules to CD8 T cells. ...

See Also

• MHC
• T cell
• Antigen_presenting_cell

MHC may be: an abbreviation for Managed health care an abbreviation for major histocompatibility complex the US Navy hull classification symbol for Coastal Minehunter the Mile High Club This page concerning a three letter acronym is a disambiguation page — a navigational aid which lists other pages that might otherwise share... T cells are a subset of lymphocytes that play a large role in the immune response. ... An antigen presenting cell (APC) is a cell that displays foreign antigen complexed with MHC on its surface. ...

References

Goldsby, Richard (2002). Immunology (5th ed.). W.H. Freeman.