Final answer:
MHC class I molecules present endogenously derived peptides, typically from proteins within the cell or from intracellular pathogens. These peptides then signal to cytotoxic T cells if a cell is infected and should be destroyed. In contrast, MHC class II molecules are associated with presenting extracellularly derived peptides.
Step-by-step explanation:
MHC class I molecules are responsible for presenting endogenously derived peptides to the immune system. These peptides are typically sourced from the normal protein turnover of a cell or from proteins produced by intracellular pathogens like viruses. Once these peptides are generated, they are bound by MHC class I molecules inside the cell and then transported to the cell surface, where they can be recognized by cytotoxic T cells. If a cell is infected with a pathogen, presenting these foreign peptides results in the activation of an immune response that targets the infected cell for destruction. MHC class II, on the other hand, presents peptides derived from extracellular pathogens, including many bacteria, parasites, and fungi, which are phagocytosed by professional antigen-presenting cells and bound to MHC class II molecules for presentation to helper T cells.