Final answer:
Memory B cells differentiate into plasma cells upon reexposure to a pathogen, playing a crucial role in the secondary immune response and rapidly producing antibodies to prevent the establishment of the infection.
Step-by-step explanation:
Upon reexposure to a pathogen, a memory B cell can differentiate into a plasma cell. This process is a key component of the body's secondary immune response. Memory B cells are long-lived cells that remember the pathogen they were created to defend against. When they encounter the same pathogen again, they can rapidly differentiate into plasma cells without the need for interaction with antigen-presenting cells (APCs) or helper T (TH) cells. These new plasma cells then produce antibodies at a much higher rate than during the primary response, often preventing the infection from becoming established. This is why memory B cells are essential for long-term immunity.