Final answer:
Memory T cells and B cells survive after an infection to provide a rapid and enhanced response if the pathogen is encountered again, often stopping reinfection before it becomes symptomatic.
Step-by-step explanation:
After an infection has been cleared, most T cells and B cells die off, but a subset of these cells known as memory T cells and memory B cells remain in the body. These memory cells retain a 'recollection' of the pathogen, holding onto specific receptors highly affine to the previously encountered antigen. Therefore, if the same pathogen invades the body again, these memory cells quickly proliferate, mounting a rapid immune response without the need for antigen presentation by APCs or activation by helper T cells. This accelerated response, led predominantly by memory T cells differentiating into cytotoxic T lymphocytes (CTLs) and memory B cells converting into plasma cells, is more efficient and can often neutralize an infection before it becomes symptomatic.
Furthermore, memory T cells have the role of assisting other parts of the immune system. Helper T cells, for example, secrete cytokines that control the activity of other immune cells and are essential for coordinating an immune response. They too can form memory cells that aide in a faster and stronger response upon reexposure to the same antigen.