The blood–thymus barrier regulates exchange of substances between the circulatory system and thymus, providing a sequestered environment for immature T cells to develop.
The barrier also prevents the immature T cells from contacting foreign antigens (since contact with antigens at this stage will cause the T cells to die by apoptosis).
The barrier is formed by the continuous blood capillaries in the thymic cortex, reinforced by type 1 epithelial reticular cells (sometimes called thymic epithelial cells) and macrophages.
The existence of this barrier was first proposed in 1961 and demonstrated to exist in mice in 1963.
[1]