Cytotoxic T cells therefore clean up tissues in parallel to the way that antibodies produced by B cells clean up the blood. A second type of effector T cell, effector helper T cells , has a separate role. Helper T cells act by stimulating effector B cells to produce antibodies. These, by the way, are the cells that are infected by the HIV virus. Like memory B cells, memory cytoxic T cells and memory helper T cells do not participate in the immediate response to the pathogen, but instead remain in the circulatory system.
Memory cells persist in the circulation for decades.
They are the basis of the secondary immune response . When activated by reappearance of the same antigen that triggered their initial production, they are in turn activated. They clone themselves to produce effector and memory cells. This means that the secondary immune response is then always available to protect you from that pathogen, and it typically works so well you usually don't even know that you were ever re-exposed. There is substantial evidence that vaccination works to control human disease.