Tc cells are CD8 positive, and have MHC I restricted TCRs on their surface. The activation of naive Tc cells depends on Th help and is summarized in figure 14-1 . The activation of memory CTL is not dependent on Th cells and is summarized in figure 14-2 . The study of how Th avoid self reactivity continues to be an active area of research in immunology. The ability to avoid reacting against self is called self-tolerance .
Several mechanisms operate to provide Th self-tolerance.
Tolerance by clonal deletion in the thymus during negative selection. Tolerance by clonal anergy in the peripheral lymphoid organs. If an antigen is presented on a resting macrophage, it will not stimulate the bound Th cell because B7 will not be present to deliver a co-stimulatory signal. In addition, subsequent encounters with APC will also fail to activate this Th cell. This is called clonal anergy, meaning that the Th cell is still alive and circulating, but not activated.