Final answer:
Exogenous antigens are brought into the endomembrane system of the cell through receptor-mediated endocytosis in cross-presentation. The vesicle containing the antigen fuses with vesicles from the Golgi complex, which have pre-formed MHC class II molecules. The mechanism by which exogenous antigens enter the endoplasmic reticulum in cross-presentation is not fully understood.
Step-by-step explanation:
In cross-presentation, exogenous antigens are brought into the endomembrane system of the cell through receptor-mediated endocytosis. These antigens are typically characteristic of bacteria, parasites, and fungi that do not replicate inside the cell's cytoplasm. The resulting vesicle fuses with vesicles from the Golgi complex, which contain pre-formed MHC class II molecules. After fusion and association of the antigen with MHC, the new vesicle makes its way to the cell surface.
Cross-presentation is primarily a function of dendritic cells and involves phagocytosis of the pathogen. Dendritic cells, not directly infected by the pathogen, use this mechanism to present antigens on MHC class I molecules rather than MHC class II molecules. The exact mechanisms of how exogenous antigens are brought into the endoplasmic reticulum in cross-presentation are not yet well understood.