One of the major characteristics of the acquired-immunity system is that it exhibits memory. Although the system is slow to respond the first time it encounters an antigen, it is much quicker the next time. The process of generating T-cell memory involves the death of most of the T cells that were generated by the first infection with a particular antigen. Only a small percentage (5%–10%) of the original cells survive as memory cells.
Still, this represents a much larger number than was present before the initial encounter with the antigen. These memory cells have a higher reproductive rate even in the absence of antigen than does a naïve T cell (one that has never encountered the antigen).
Several of the interleukins play key roles in these processes. Interleukin 7 is involved in maintenance of naïve killer T cells at low levels. When stimulated by antigens, the proliferation of TC cells is stimulated by interleukin 2. Memory TC cells, on the other hand, are maintained by interleukin 15.