Which of the following describes the role of memory B cells upon re-exposure to an antigen?

• A. They provide immediate cytotoxic killing of infected cells
• B. They phagocytose pathogens
• C. They rapidly differentiate into plasma cells and secrete antibodies upon re-exposure
• D. They promote coagulation to prevent bleeding

Answer: (C)

Memory B cells lie in wait for a familiar antigen and respond much more quickly the second time around. When re-exposed, they are reactivated and rapidly differentiate into plasma cells, which then secrete large amounts of high-affinity antibodies. This rapid, robust antibody production is the hallmark of the secondary humoral immune response and explains why booster exposures produce a stronger and faster antibody response than the first encounter. The other options describe functions of other immune cells—cytotoxic killing by CTLs, phagocytosis by macrophages/neutrophils, and coagulation—which are not roles of memory B cells.