T cells play a major role in the immune system. While developing in the thymus, useful T cells are selected for expansion and harmful ones are deleted or inactivated. During this process, it is the T cells themselves which are doing the learning as opposed to the thymus doing the teaching. Both clonal deletion and clonal anergy are mechanisms of tolerance, the former being thymic, and thus central, while the latter is peripheral. Following a signal from the T cell receptor, anergy or deletion is triggered, if there are no co-stimulatory factors. Here, the outcome is determined by binding intensity. However, if the signal from the T cell receptor is accompanied by the required additional co-stimulatory signals, then the T cell is rescued from apoptosis and proliferates.