Final answer:
Memory cells are antigen-specific lymphocytes that provide long-term immunity by rapidly becoming effector cells upon re-exposure to a previously encountered pathogen, differentiating into plasma cells that secrete large quantities of antibodies.
Step-by-step explanation:
Those that do not become plasma cells become memory cells, how long-term immunity is possible. Memory cells are a critical component of the adaptive immune system. They are antigen-specific B or T lymphocytes that do not differentiate into effector cells during the primary immune response but can become effector cells upon re-exposure to the same pathogen. This mechanism allows for a rapid response to pathogens the body has previously encountered. Memory cells, contrasted with effector cells that die off after their function is concluded, persist in the body to provide long-term immunity. During the initial immune response, when a B cell binds to an antigen, it either becomes a plasma cell that produces antibodies or a memory B cell. Memory B cells remain inactive until they encounter the same antigen again, and upon re-infection, they differentiate rapidly into plasma cells. These plasma cells output a significantly greater amount of antibodies than during the primary response, which can effectively prevent the pathogen from establishing an infection.