Final answer:
Extracellular antigens are internalized by receptor-mediated endocytosis and presented on the cell surface bound to MHC class II molecules after fusion with Golgi vesicles.
Step-by-step explanation:
Extracellular antigens from organisms such as bacteria, parasites, and fungi, which do not replicate inside a host cell's cytoplasm, interact with the endoplasmic reticulum (ER) through a process involving the cell's endomembrane system. This interaction begins when the antigens are internalized via receptor-mediated endocytosis, a process by which cells absorb specific molecules present in their surrounding environment. The internalized vesicle, carrying the antigen, fuses with vesicles from the Golgi apparatus that contain MHC class II molecules. Once this fusion occurs, the antigen becomes associated with the MHC class II molecules, and the newly formed vesicle travels to the cell surface to present the antigen. This is a crucial step for the initiation of an immune response by professional antigen-presenting cells.
During the immune response, T cell receptors (TCR) on helper T cells recognize extracellular antigens presented by MHC class II molecules. In contrast, cytotoxic T cells use their TCRs to interact with intracellular antigens presented by MHC class I molecules, which are expressed by nearly all cells in order to stimulate a T cell immune response against intracellular pathogens, such as viruses. The intracellular pathway also involves the synthesis of proteins that begin in the rough ER, where the protein's signal peptide emerges and interacts with the ER membrane during translation. If a peripheral membrane protein is synthesized inside the ER, where it ends up in the plasma membrane depends on the topology during synthesis and later sorting processes.