Final answer:
Intracellular peptides are loaded onto MHC class I molecules. These peptides are generally from viruses and other similar intracellular pathogens, and the process leads to the peptides being presented to cytotoxic T cells for immune response activation.
Step-by-step explanation:
Peptides that are brought in from inside the cell (intracellular) are loaded onto MHC class I molecules. Intracellular antigens, such as viruses that replicate inside the host cell, are processed in the cytosol by a proteasome. They are then transported into the endoplasmic reticulum via the transporter associated with antigen processing (TAP) system, where they bind to MHC class I molecules. These MHC class I-antigen complexes are then transported to the cell surface in a vesicle, where they can be presented to cytotoxic T cells, which may lead to an immune response that targets the infected cells for destruction. Unlike MHC class I, MHC class II molecules present extracellular antigens and are found on professional antigen-presenting cells such as macrophages, dendritic cells, and B cells. These antigens are typically associated with pathogens that do not replicate inside the cell's cytoplasm, like many bacteria, parasites, and fungi. They are processed differently through receptor-mediated endocytosis into the endomembrane system and then presented on the cell surface via MHC class II molecules to helper T cells.