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The repertoire of T cells is shaped by both positive and negative selection

Andrew Bushell and Kathryn J. Wood

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Figure 2. The repertoire of T cells is shaped by both positive and negative selection. T-cell receptors (TCRs) are generated by random gene rearrangements. In the thymus, those T cells with TCRs that are unable to recognise self major histocompatibility complex (MHC) molecules are allowed to die, because they will not play a useful role in the immune system. The T cells that remain are ‘positively selected’ for further development. Many of these positively selected T cells recognise self MHC molecules with such a high avidity that they would be autoreactive and capable of causing autoimmune disease. These T cells are deleted (i.e. they are killed) in the thymus by negative selection. The remainder (probably <5% of those that originally entered the thymus from the bone marrow) are allowed to mature and enter the peripheral immune system (fig002abo).

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