The skin is a dynamic organ that contains different cells which contains elements of the innate and the adaptive immune systems which are activated when the tissue is under attack by invading pathogens.
Keratinocytes produce TNFα and IL-1 which act on the Langerhans cells, inducing an increase of the expression of histocompatibility complex and cytokine secretion.
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.
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.
Thus skin commensals exert their effect by enhancing IL-1 signaling and amplifying responses according to local inflammatory milieu.
Recent studies have demonstrated that specific components of the microbiota, as well as their metabolites, selectively promote the activation and the expansion of different T cell subsets under normal and/or pathological conditions.
The connection between the innate and the adaptive system is driven in this case by the production of alarmins S100A8 and S100A9 known to elicit microbicidal responses and as potent chemoattractants for neutrophils.
This shows that the skin immune system is a highly dynamic environment that can be rapidly and specifically remodeled by certain commensals.