In the thymus, T-cells that recognize self-targets are negatively selected and die, while those that do not recognize self-targets are positively selected.

“So they they've have are making these sensors at largely at random and then in the thymus they get cold they get selected and they the ones that by accident are generated that recognize something that is supposed to be in your body if if the T- cell engages a natural target in the thymus those cells will die and so what emerges from the thymus should be and this is not perfect process but should be things that have are have emerged at random but then are selected to remove things that recognize your own body targets.”