Figure 1: Antigen processing pathways. Direct presentation involves the processing of endogenously synthesized antigens (route A) by the proteasome (I) to break them down into smaller fragment (polypeptides) that are transported through the transporter associated with antigen processing (TAP) into the endoplasmic reticulum (II) for loading onto newly synthesized MHC class I molecules. These peptide-loaded MHC class I molecules are then transported through the Golgi (III) to the cell surface for presentation to CD8+ T cells. Considering the major route for antigen cross-presentation (route B), exogenous antigens are internalized in the endosome/phagosome before they are released into the cytosol and degraded by the proteasome to be presented on MHC class I molecules.