**4. Blood-thymus barrier**

It is a physical barrier formed by endothelial cells, epithelial reticular cells, and macrophages. Its function is to prevent developing lymphocytes from the exposure of blood borne antigen [8]. This barrier provides tremendous environment for the substance exchange between vasculature and the thymus also help maturation of the immature thymocytes. Macrophages present outside the capillaries prevent the interaction of the substances that are transported in the blood vessels with the developing T cells in the cortex. Matured T cells leave the thymus gland through the blood vessels and colonize in the lymph node, spleen, and lymphatic tissue of the organism [11].
