Final answer:
Effector cells are crucial for immediate immune response and have a short lifespan, dying off once the infection is cleared. Memory cells, however, persist in the bloodstream for years or a lifetime, ready to mount a rapid defense if the same pathogen is encountered again, providing immunity.
Step-by-step explanation:
Within the adaptive immune system, two distinct types of cells are produced during a primary immune response: effector cells and memory cells. Effector T cells have a short lifespan and are programmed to perform immediate functions in response to the pathogen. They proliferate rapidly and are instrumental in clearing an infection. However, once their job is done and the pathogen is cleared, these cells undergo apoptosis, typically within a few days.
In contrast, memory cells are long-lived cells that remain in the bloodstream after an infection has been conquered. These cells can persist for years or even decades. Their primary role is to 'remember' antigens, ensuring that if the same pathogen invades the body again, the immune system can rapidly mount a defense against it. Memory cells can differentiate into effector cells upon re-exposure to the antigen without the need for activation by antigen-presenting cells (APCs), leading to a quicker, more potent immune response.
Immunity to a disease is due to the presence of these memory cells, which enable a rapid secondary immune response that often prevents the pathogen from causing any symptoms. If the pathogen is never encountered again, memory cells might gradually die off, having never turned into effector cells.