Brain vessels enter the central nervous system through holes in the meningeal layers. The cells in the blood vessel walls are joined tightly to one another, forming the blood–brain barrier, which blocks the passage of many toxins and pathogens (though at the same time blocking antibodies and some drugs, thereby presenting special challenges in treatment of diseases of the brain). Neuroanatomists usually divide the vertebrate brain into six main regions: the telencephalon (cerebral hemispheres), diencephalon (thalamus and hypothalamus), mesencephalon (midbrain), cerebellum, pons, and medulla oblongata. Each of these areas has a complex internal structure. Some parts, such as the cerebral cortex
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