Figure 1.

Antigen-processing pathways in the cell. Left: intracellular pathway. Protein is cleaved into oligopeptides in the proteasome, the peptides enter the endoplasmic reticulum (ER) via TAP protein and bind to MHC class I, and the complex peptide–MHC protein is presented on the cell surface. Right: extracellular pathway. Protein is endocytozed, cleaved into oligopeptides in the endosome, bound to MHC class II protein and presented on the cell surface. In the ER, MHC class II molecules are adjoined to a specific peptide, known as invariant chain (Ii). It blocks the binding cleft of the MHC molecule, thereby preventing the binding of endogenous peptides. In the endosome, the Ii is initially cleaved to CLIP peptide, and is then replaced by an exogenous peptide. The process is facilitated by the HLA-DM molecule.