How are alpha beta T cells positively selected in the thymus?

Thymus-derived (T) lymphocytes have the potential to express antigen receptors (TCR) that can recognize both self, as well as foreign antigens as they appear on the cell surface. In the thymus, positive selective allows the maturation of T cells that are able to see foreign antigens in conjunction with molecules encoded by genes of the major histocompatibility complex (MHC), whereas negative selection deletes auto-aggressive T cells. Control of T cell development is the only known function of the thymus. Therefore, it has been argued that recognition events responsible for selection of the T cell repertoire are guided by unique cellular interactions in the thymus. Here, we will show that positive selection can also occur on non-thymic cells. We will also argue that positive selection is not dependent on unique thymic accessory cells (AC) function. In other words, restricted recognition is not taught rather thymocytes learn it by themselves.