Final answer:
Peptides are generated from intracellular pathogens via proteasomal degradation, TAP transport, and MHC class I presentation, while extracellular pathogens are presented by APCs through phagocytosis, lysosomal degradation, and MHC class II presentation.
Step-by-step explanation:
Peptide Generation from Pathogens
The immune system generates peptides from both intracellular and extracellular pathogens to present to immune cells, a process vital for the identification and elimination of these pathogens. For intracellular pathogens, such as viruses and some bacteria, the peptides are typically generated within infected cells by the proteasome after the pathogens are recognized and degraded. This is followed by the transport of peptides into the endoplasmic reticulum by transporters associated with antigen processing (TAP), where they are loaded onto MHC class I molecules and presented on the cell surface to cytotoxic T cells.
On the other hand, extracellular pathogens, like most bacteria and parasites, are taken up by antigen-presenting cells (APCs) such as dendritic cells through phagocytosis. The pathogens are then degraded in lysosomes, and the resulting peptides are loaded onto MHC class II molecules and presented on the surface of APCs to helper T cells.
Both processes are essential for the adaptive immune response, allowing specific immune cells to recognize and respond to pathogenic antigens effectively.