blood–thymus barrier blood–thymus barrier 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.