Cerebellum cerebellum may be absent. The inherited neurological disorders Machado–Joseph disease, ataxia telangiectasia, and Friedreich's ataxia cause progressive neurodegeneration linked to cerebellar loss. Congenital brain malformations outside the cerebellum can, in turn, cause herniation of cerebellar tissue, as seen in some forms of Arnold–Chiari malformation. Other conditions that are closely linked to cerebellar degeneration include the idiopathic progressive neurological disorders multiple system atrophy and Ramsay Hunt syndrome type I, and the autoimmune disorder paraneoplastic cerebellar degeneration, in which tumors elsewhere in the body elicit an autoimmune response that causes neuronal loss in the cerebellum. Cerebellar atrophy can result from an acute
Antigen transfer in the thymus in thymic medulla, namely clonal deletion ("recessive tolerance") and T Regulatory cells selection ("dominant tolerance") which force autoreactive T cells to apoptosis or skew them into suppressor T regulatory cells (TRegs), respectively, in order to protect body against manifestations of autoimmunity. These processes are mediated especially by unique subset of stromal cells called Medullary thymic epithelial cells (mTECs) via presentation of Tissue restricted antigens (TRAs) that represent self tissues from almost all parts of the body. mTECs are not only capable to present TRAs as efficient APCs. They are also potent in production of these TRAs via unique process called