Immunology | Immune system| Immunity

The skin is an important anatomic barrier to the external environment,and its large surface area makes this tissue important in nonspecific(innate)defenses.The epidermal (outer)layer of the skin is composed largely of specialized epithelial cells called keratinocytes.These cells secrete a number of cytokines that may function to induce a local inflammatory reaction.In addition,keratinocytes can be induced to express class II MHC molecules and may function as antigen-presenting cells.Scattered among the epithelial-cell-matrix of the epidermis are Langerhans cells,a type of dendritic cell,which internalize antigen by phagocytosis or endocytosis.The Langerhans cells then migrate from the epidermis to regional lymph nodes,where they differentiate into interdigitating dendritic cells.These cells express high levels of class II MHC molecules and function as potent activators of naive Helper cells.
 
The epidermis also contains so-called intraepidermal lymphocytes.These are similar ti the intraepithelial lymphocytes of MALT in that most of them are CD8 plus cells,which have limited diversity for antigens that enters through the skin and some immunologist believe that they may play a role in combating antigens that enter through skin.The under-laying dermal layer of the skin contains scattered CD4 plus and CD8 plus T cells and macrophages.Most of these dermal T cells were either previously activated cells or are memory cells.