Antigens from<unk> d regions have been found to interact with T cells in an unusual way: inducing tolerance of normally rejected stimuli.<unk>  has emerged as an active rather than a passive process. Physical structures surrounding<unk> d sites cause a lack of lymphatic drainage, limiting the<unk>  system's ability to enter the site. Other factors that contribute to the maintenance of<unk>  include: * low expression of classical MHC class Ia molecules * expression of<unk> regulatory nonclassical, low polymorphic class Ib MHC molecules * increased expression of surface molecules that inhibit complement activation * local production of<unk> suppressive cytokines such as TGF-β * presence of neuropeptides * constitutive expression of Fas ligand that controls the entry of Fas-expressing lymphoid cells. The nature of isolation of<unk> logically<unk> d sites from the rest of the body's<unk>  system can cause them to become targets of autoimmune diseases or conditions, including sympathetic ophthalmia in the eye.
Immune privilege
Antineoplastic resistance
Extravasation 
Tumor initiation
Immune tolerance
Clonal hematopoiesis
Tumor budding
Tissue expansion