Final answer:
Antigen entry into cells generally occurs by endocytosis for extracellular pathogens presented via MHC class II molecules, and by MHC class I presentation for intracellular pathogens such as viruses that are processed by the proteasome and transported by TAP system to interact with MHC class I molecules.
Step-by-step explanation:
There are two possible routes of antigen entry into cells that can be discussed: endocytosis and MHC class II antigen presentation for extracellular antigens, and MHC class I antigen presentation for intracellular antigens.
Endocytosis is a process by which cells absorb external material by engulfing it in a pocket of cell membrane that pinches off inside the cell to form a vesicle. Extracellular pathogens, like some bacteria and parasites, can be brought into the endomembrane system of the cell through receptor-mediated endocytosis, where they will eventually be presented on the cell surface bound to MHC class II molecules.
On the other hand, intracellular pathogens such as viruses, which replicate inside the cell's cytoplasm, are processed by the proteasome and conveyed into the endoplasmic reticulum by the transporter associated with antigen processing (TAP) system. Here, antigens interact with MHC class I molecules and are then transported to the cell surface by a transport vesicle, where they can be recognized by T cells.
In the context of an extracellular bacterial infection, the MHC class II pathway of antigen presentation would typically be used to present antigens from the pathogen. This is because the extracellular antigens are endocytosed and presented by professional antigen-presenting cells on their surface via MHC class II molecules, where they can then be recognized by T cells.