[Skin as an immune system]

Skin consists of three structurally and functionally distinct compartments containing resident and nonresident cells. These cells cooperative with humoral pathway of immune system in defence of healthy skin. Resident and nonresident cells are able to initiate inflammatory or immune processes of skin, although interstitial reactions (activation by bacteria, immunoglobulin or complement) also take place in such processes. In normal conditions nonresident cells can migrate through vascular endothel, however, PMN granulocytes and B lymphocytes cannot. Resident and nonresident cells of skin are capable of exerting a wide range of immunomodulatory effects, among them keratinocytes are of distinguished significance producing arachidon metabolites and IL-1 as well. Activated skin cells might induce chemotactic migration of distinct white blood cells normally absent or only a few cells being present in healthy skin. Cell migration into the interstitial space of skin is mediated by adhesion molecules expressed on cell surface of migrating cells, vascular endothel cells and on keratinocytes. Composition and density of adhesion molecules vary by type of stimuli, therefore various cytokines might induce distinct reactions mediated by certain population of cells. In addition, initiation and progress of immune- or inflammatory reactions are determined by the involved cell-populations as well. Normal regulation provides appropriate control and termination of the reaction, however, uncontrolled regulation results in development of pathological state.