The skin immune system
The horny layer of the skin is able both to prevent the loss of fluid and electrolytes, and to stop the penetra-tion of harmful substances. It is a dry mechani-cal barrier from which contaminating organisms and chemicals are continually being removed by washing and desquamation. Only when these breach the horny layer do the cellular components, described below, come into play. The skin is involved in so many immunological reactions, seen regularly in the clinic (e.g. urticaria, allergic contact dermatitis, psoriasis, vasculitis), that a special mention has to be made of the peripheral arm of the immune system based in the skinathe skin immune system (SIS).
The idea of an SIS as a functionally independent immunological unit is helpful. It includes the cuta-neous blood vessels and lymphatics with their local lymph nodes and contains circulating lymphocytes and resident immune cells. Although it is beyond the scope of this book to cover general immunology, this section outlines some of the intricate ways in which antigens are recognized by specialized skin cells, mainly the Langerhans cells, and how antibodies, lymphocytes, macrophages and polymorphs elicit inflammation.