Final answer:
Populations of long-lived T or B cells that have been stimulated by antigens are called memory cells. They allow the immune system to 'remember' the pathogen and mount a faster and stronger immune response upon reinfection.
Step-by-step explanation:
Populations of long-lived T or B cells that have been stimulated by antigens are called memory cells. These memory cells are formed after an initial infection and can persist in the body for many years. They allow the immune system to 'remember' the pathogen and mount a faster and stronger immune response upon reinfection.