Này lưu thông ưu đãi của tế bào lympho vào da là một phần đặc biệt của hệ thống miễn dịch da và phản ánh một lợi thế chọn lọc cho cơ thể để lưu hành các tế bào lympho phản ứng với kháng nguyên có nguồn gốc từ bề mặt da và da. | 28 CHAPTER 2 stimulated their production. This preferential circulation of lymphocytes into the skin is a special part of the skin immune system and reflects a selective advantage for the body to circulate lymphocytes that react to skin and skin surface-derived antigens. Elicitation challenge phase Fig. When a T lymphocyte again encounters the antigen to which it is sensitized it is ready to react. If the antigen is extracellular as on an invading bacterium toxin or chemical allergen the CD4 T-helper cells do the work. The sequence of antigen processing by the Langerhans cell in the elicitation reaction is similar to the sequence of antigen processing during the induction phase described above that leads to the induction of immunity. The antigens get trapped by epidermal Langerhans cells or dermal dendritic cells which process the antigen intracellularly before re-expressing the modified antigenic determinant on their surfaces. In the elicitation reaction the Langerhans cells find appropriate T lymphocytes in the dermis so most antigen presentation occurs there. The antigen is presented to CD4 T cells which are activated and produce cytokines that cause lymphocytes polymorphonuclear leucocytes and monocytes in blood vessels to slow as they pass through dermal blood vessels to stop and emigrate into the dermis causing inflammation Fig. . Helper or cytotoxic lymphocytes help to stem the infection or eliminate antigen and polymorphonuclear leucocytes engulf antigens and destroy them. The traffic of inflammatory cells in the epidermis and dermis is determined not only by cytokines produced by lymphocytes but also by cytokines produced by injured keratinocytes Fig. . For example keratinocyte-derived cytokines can activate Langerhans cells and T cells and IL-8 produced by keratinocytes is a potent chemotactic factor for lymphocytes and polymorphs and brings these up into the epidermis. LEARNING POINTS 1 Many skin disorders are good examples of an immune .
