The blood–thymus barrier protects developing lymphocytes in the thymus from exposure to antigens.
Lymphocytes reaching the thymic cortex are prevented from contact with antigen by a physical barrier called the blood– thymus barrier (Fig. 14.27). The following components constitute the blood–thymus barrier between the T cells and the lumen of cortical blood vessels, from the lumen outward:  The endothelium lining the capillary wall is of the continuous type with occluding junctions. It is highly impermeable to macromolecules and is considered a major structural component of the barrier within the cortical parenchyma. The underlying basal lamina of endothelial cells and occasional pericytes are also part of the capillary wall.
FIGURE 14.27 • Schematic diagram of the blood–thymus barrier. The blood–thymus barrier consists of three major elements: (1) capillary endothelium and its basal lamina, (2) perivascular connective tissue space occupied by macrophages, and (3) type I epithelioreticular cells with their basal lamina. The perivascular connective tissue is enclosed between the basal lamina of the epithelioreticular cells and the endothelial cell basal lamina. These layers provide the necessary protection to the developing immature T cells and separate them from mature immunocompetent lymphocytes circulating in the bloodstream.
 Macrophages residing in the surrounding perivascular connective tissue may phagocytose antigenic molecules that escape from the capillary lumen into the cortical parenchyma.
 Type I epithelioreticular cells with their occluding junctions provide further protection to the developing T cells. The epithelioreticular cells surround the capillary wall in the cortex; with their basal lamina, they represent another major structural component of the blood–thymus barrier.
The thymus is the site of T-cell education.
During fetal life, the thymus is populated by multipotential lymphoid stem cells that originate from the bone marrow and are destined to develop into immunocompetent T cells. Stem cell maturation and differentiation into immunocompetent T cells is called thymic cell education (Fig. 14.28). This process is characterized by the expression and deletion of specific surface CD antigens.
immunocompetent T cells is accomplished by the expression and deletion of specific surface CD DO NOT selection antigens. The CFU-L stem cells enter the medulla PASS of the thymus via a postcapillary venule and then type migrate to the periphery of the thymic lobule. The
II erc presence of CD2 and CD7 molecules on the cell surface indicates an early stage of differentiation.
This is followed by expression of the CD1
The expression of CD2 and CD7 molecules on the T-cell surface indicates an early stage of differentiation (doublenegative stage). The double-negative term refers to lack of both CD4 and CD8 molecules. This early stage is followed by the expression of the CD1 molecule, which indicates the middle stage of T-cell differentiation. As maturation progresses, the T-cells express TCRs, CD3, and both CD4 and CD8 molecules. This is the double-positive stage of T-cells differentiation. These cells are then presented with selfand foreign antigens by type II and III epithelioreticular cells. If the lymphocyte recognizes self-MHC molecules and selfor foreign antigen, it will survive, a process called positive selection. If not, the cell will die. Cells that pass the positive-selection test leave the cortex and enter the medulla. Here, they undergo another selection process in which cells that recognize self-antigen displayed by self-MHC are eliminated, a process called negative selection. The cells that survive become either