The transport of MHC molecules is largely determined by the completion of their folding. This critical step is dependent upon the acquisition of immunogenic peptides able to stabilize the structure of the terminal part of the molecule which is exposed to the T cell receptor. The association with chaperone-like molecules allows the loading to take place in specific compartments: ER for MHC class I molecules, and endocytotic pathway for MHC class II molecules. Different affinities for different chaperone molecules split antigen presentation into two different pathways: endogenous (for MHC class I molecules), and exogenous (for MHC class II molecules). The MHC class II chaperone molecule is the invariant chain. This polypeptide regulates the transport of MHC class II molecules towards the endosomal pathway. It also governs the accessibility of the peptide binding groove of MHC class II molecules.