Final answer:
Extracellular pathogens enter B cells via receptor-mediated endocytosis, leading to antigen presentation and immune response activation.
Step-by-step explanation:
When extracellular pathogens are entering B cells, they typically use a method of entry known as receptor-mediated endocytosis. In this process, the pathogen binds to specific receptors on the surface of the B cell, leading to the engulfment of the pathogen into the cell inside a vesicle.
Once inside, the pathogen is processed and fragments of it are displayed on the B cell's surface in association with MHC class II molecules. This allows for the antigen presentation pathway to become activated, which is essential for triggering a subsequent immune response.