Positive selection occurs in the thymic cortex.

Double-positive thymocytes (CD4/CD8) move deep into the thymic cortex where they are presented with self-antigens.

Thymocytes that survive positive selection migrate towards the boundary of the thymic cortex and thymic medulla.

The barrier is formed by the continuous blood capillaries in the thymic cortex, reinforced by epithelial reticular cells and macrophages.

Rentea R, Lyon E, Hunter R: Biologic properties of iscador: a Viscum album preparation I. Hyperplasia of the thymic cortex and accelerated regeneration of hematopoietic cells following X-irradiation.

Early during T-cell development in the thymic cortex, thymocytes bear both of the T cell surface glycoproteins CD4 and CD8 and express the α and β heterodimer of the TCR [ 4 ] .