Antigen presentation

Antigen presentation describes a vital immune process. Because T cells only recognise antigens displayed on the cell surfaces, an antigen-presenting complex is needed for detecting infectious cells. After infection with viruses or bacteria, the cells present endogenous peptide fragments derived from the pathogen by molecules on the cell surface. There are two types of MHC, depending on the source of the antigens: MHC class I molecules (MHC-I) binds peptides from the cytosol, while peptides generated in vesicles are bound to MHC class II (MHC-II). T cells can recognise among ten to hundreds of thousands peptides because each MHC molecule can bind a different range of peptides.

Cytotoxic T cells regularly patrol body cells to maintain the organism healthy. Whenever they encounter signs of disease, caused for example by the presence of viruses or intracellular bacteria, they initiate processes to destroy the potentially harmful cell. To effect recognition, all nucleated cells in the body (along with platelets) display class I (MHC-I molecules). Antigens generated endogenously within the cells (DAMPs) are bounded to MHC-I molecules and presented on the cell surface. This antigen presentation pathway enables the immune system to detect transformed or infected cells displaying peptides from modified-self or foreign proteins.

In the presentation process, these proteins are mainly degraded into small peptides by cytosolic proteases in the proteasome, but there are also other cytoplasmic proteolytic pathways. Then, the transporter associated with antigen processing (TAP) translocates the cytosolic peptides into the ER lumen in an ATP-dependent transport mechanism. There are several ER chaperones involved in MHC-I assembly, as calnexin, clareticulin and tapasin. Because of the association between TAP and MHC-I, the peptides are loaded to MHC-I molecules. After releasing from tapasin, peptide-MHC-I complexes (pMHC-I) exit the ER and are transported to the cell surface by exocytic vesicles.

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