Final answer:
Intracellular antigens are typically from viruses, processed by the proteasome, presented to the endoplasmic reticulum, and bound to MHC class I molecules for recognition by cytotoxic T cells.
Step-by-step explanation:
How do intracellular antigens interact with the ER? Intracellular antigens typically arise from pathogens like viruses that replicate within a host cell. These antigens are processed in the cytosol by the proteasome and then transported into the endoplasmic reticulum (ER) via the transporter associated with antigen processing (TAP). Inside the ER, they bind to MHC class I molecules. This complex of antigen plus MHC class I is then conveyed inside a transport vesicle to the cell surface, where it can be recognized by cytotoxic T cells. This cellular mechanism is crucial for the immune response against most classes of intracellular pathogens, especially viruses, that infect tissue cells.