The immune system is an adaptive defence system capable of specifically recognizing and eliminating an apparently limitless variety of foreign invaders. The enormous diversity of the antigen-specific receptors on B and T lymphocytes, the key players in the adaptive immune response, is generated by random rearrangement of the respective genes. The main challenge of such a defence system is to have as broad a T and B cell repertoire as possible in the absence of autoreactivity. Several tolerance mechanisms are operating in parallel under physiological conditions to silence T cells either during their development in the thymus or in the periphery.
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