<P> The skin acts as a barrier, a kind of sheath, made of several layers of cells and their related glands . The skin is a dynamic organ that contains different cells which contains elements of the innate and the adaptive immune system which are activated when the tissue is under attack by invading pathogens . Shortly after infection, the immune adaptive response is induced by dendritic cells (Langerhans cells) present in the epidermis; they are responsible to capture, process, and present the antigens to T lymphocytes in local lymphoid organs . As result, T lymphocytes express the CLA molecule . Lymphocytes move to the epidermis where they reside as memory T cells, they will thus be activated and will trigger an inflammatory response . Dysregulation of these mechanisms is associated with inflammatory diseases of the skin . </P> <P> Some humoral and cellular components of the skin pass through the vessel lymph to get to the circulation . This circulation net has a big importance, it's the way of direct communication between the specific site of the skin and the lymph cells found inside the lymph node and the systematic tissues . The epidermis antigens are connected with some cells of the skin . Among them there are the APC, antigen presenting cells (Langerhans, dentritic and cutaneous). They capture the antigen, they process it and they present it on their surface as being associated with the MHC - II . Keratinocytes produce TNFα and IL - 1 which act on the Langerhans cells, inducing an increase of the expression of histocompatibility complex and cytokine secretion . Moreover, they induce their migration from the skin to the paracortical areas of the lymph nodes . Once there, these cells can provide the necessary stimulus for the lymphocytes T, who will proliferate and express the cutaneous receptor recruitment and to various chemo attractants that promote the accumulation of dermal micro vascular endothelial cells of inflamed skin to finally enter the skin tissue . Once the activated lymphocytes arrive, they get in contact with the antigen, they proliferate and develop their effector functions in order to neutralize or eliminate the pathogen . The Langerhans cells promote and permit the start of the cellular immune response of lymphocytes through the skin and are recruited from the peripheral blood . Antigen presentation may occur in peripheral lymphoid tissues . </P> <P> The Langerhans cells, once they are activated, they rapidly migrate to the lymph nodes where they will accumulate in the paracortex and show the antigen of the skin to the lymph nodes via efferent lymph vessels . The Langerhans cells induce a vast proliferation of the naïve lymphocytes T and they participate in the immunoestimulation phase of the immune response, converting the lymphocytes in T helper cells . Recently, it has been shown that Langerhans cells can express an antigenic peptide associated to MHC - I capable of inducing a response from the cytotoxic LT and effector functions, such as the production of cytokines . </P> <P> Skin microbiota plays an important role in tissue homeostasis and local immunity . Skin microbial communities are highly diverse and can be remodeled over time or in response to environment challenges . </P>

Analyse how the skin acts as a barrier to infection and the role of inflammation