**9.3 Sequestration of antigens into privileged sites**

Some antigens are sequestered into privileged sites away from the immune system because of physical barriers, such as tight junctions, or immunologic barriers, such as the expression of Fas ligand (FasL) or reduced MHC class I expression. Thus, antigen-presenting cells (APCs), and subsequently T lymphocytes, may never encounter these self-antigens. Therefore, immune cells remain ignorant of these antigens. At some of these sites, proinflammatory lymphocytes are controlled by apoptosis due to the expression of FasL or the secretion of cytokines such as transforming growth factor-beta (TGF-β) or interleukin (IL)-10. When T cells enter these sites, their Fas receptors interact with the FasL of these sites, and they undergo apoptosis. Privileged sites include the brain, the testes, and the anterior chamber of the eye. Transplanted tissues are most likely to survive in these privileged sites because of the tight control of proinflammatory lymphocytes.
