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Figure 19.28 Skin-associated lymphoid tissues (SALT) that initiate cutaneous immunity are located in the epidermis (E) and dermis (D) and are separated by the dermal-epidermal junction (J) and the basement membrane (BM). Dendritic cells in the dermis (DC) and Langerhans cells (LC) residing in the epidermis are the antigen-presenting cells of the skin. The dermis contains blood vessels [capillaries (CAP) and post-capillary venules (PCV)] and lymphatic channels. Afferent lymphatic ducts (AFF) carry lymph contents to lymph nodes (LN) containing lymphoreticular cells and postcapillary venules. Lymphocytes exit the LN via efferent lymphatics (EFF) into the thoracic duct (TD) and eventually into the bloodstream. (Adapted from Streilein et al., 1994, with permission of the authors and publisher.)

Figure 19.28 Skin-associated lymphoid tissues (SALT) that initiate cutaneous immunity are located in the epidermis (E) and dermis (D) and are separated by the dermal-epidermal junction (J) and the basement membrane (BM). Dendritic cells in the dermis (DC) and Langerhans cells (LC) residing in the epidermis are the antigen-presenting cells of the skin. The dermis contains blood vessels [capillaries (CAP) and post-capillary venules (PCV)] and lymphatic channels. Afferent lymphatic ducts (AFF) carry lymph contents to lymph nodes (LN) containing lymphoreticular cells and postcapillary venules. Lymphocytes exit the LN via efferent lymphatics (EFF) into the thoracic duct (TD) and eventually into the bloodstream. (Adapted from Streilein et al., 1994, with permission of the authors and publisher.)

the reaction is seen in Figure 19.29 depicting primary metabolic products in the metabolism of histidine in skin and the effect of UV radiation. Evidence for this hypothesis was seen in animals having a mutant histidine-ammonia lyase gene with less than 10% of the wild-type levels of skin urocanic acid. Such animals did not exhibit the marked UV-induced immune suppression seen in wild-type animals (cf. Noonan and De Fabo, 1992). Other studies have also implicated TNF-a in the immunosuppressive effect of UV radiation (cf. Meunier, 1999).

y8 T Cells in the Immunological Host-Tumor Relationship

While more than 90% of T cells possess a T-cell receptor with a and P chains (Figures 19.10 and 19.11), about 1% to 10% of T cells in the peripheral blood and lymphoid organs express y and 8

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