Structure, Function and Cell Biology of the Antigen- Processing Machinery

The adaptive immune system has evolved in vertebrates to eliminate invasive pathogens and malignantly transformed cells. Proteins are degraded in the cytoplasm by the proteasome. Subsequently, the peptides are transported from the cytoplasm into the lumen of the endoplasmic reticulum (ER) by the transporter associated with antigen processing (TAP) where they are loaded on MHC class I molecules. PeptideMHC complexes leave the ER for presenting the antigenic peptides on the cell surface. Cytotoxic Tcells screen the presented peptides on the cell surface and kill virus infected or transformed cells (Figure 1).