The skin is an incredibly effective barrier to the outside world – at its simplest, it keeps the outside world out and the inside in – quite literally it holds the human body together. However, the skin is not just a simple inert envelope; the barrier it provides is complex and offers physical, chemical and immunological protection.
Physical protection The analogy that is most commonly used to describe the physical barrier properties of the skin is that of a brick wall. In this instance, the layer that really is of interest is the horny layer of the epidermis. Corneocytes form the top layer of skin, these heavily keratinised cells need to be well-hydrated to form a complete barrier – they are the bricks in the wall. These bricks are ‘cemented’ with lipids. As the skin is constantly shedding, it is physically difficult for pathogenic organisms to take hold.
When intact, this brick wall is very effective at keeping out allergens and pathogens. If the barrier function is compromised (and it only has to be relatively mildly damaged through an excoriation), pathogens or allergens can penetrate leading to an infection or an allergic reaction. Moving in the opposite direction, a break in the barrier also allows moisture and lipid to be lost. A more severe break in the skin barrier, for example following a burn, can be catastrophic, indeed life threatening, as the body struggles to cope with the challenge of extensive infection and imbalanced homeostasis.
Chemical protection The surface of the skin is covered in commensal organisms. These ‘friendly’ bacteria and fungi live in harmony with humans and indeed offer a level of protection from pathogenic or disease- causing organisms. The commensals have evolved to be able to thrive in the slightly acidic environment of the skin surface (pH of 4.5–6).
Skin cells are provided with inbuilt protection from UV radiation by melanin. In the upper layers of the epidermis, melanin is scattered throughout the cells providing protection, whilst in the lower layers the protection is specifically targeted as the melanin granules form ‘umbrellas’ over the nuclei of the basal and spinous cells.
Physical and chemical barriers form part of the body’s innate defence system.
Immunological surveillance Understanding of the role that the skin plays within the immune system is ever increasing. The role of specialised cells such as Langerhans cells has already been discussed. However, other cells also have a role within immunological surveillance. For example, epidermal keratinocytes can, in certain circumstances, express immunological markers on their surface and produce cytokines which are known as signalling molecules (examples include immunomodulating agents such as interleukins) which in turn induce an inflammatory response.
Immunity is distinguished from innate nonspecific resistance in two ways. Firstly, antigens are recognised and produce a specific response, which also involves distinguishing non-self molecules. Secondly, the body remembers previous encounters with antigens so that subsequent encounters prompt more rapid and vigorous responses. Generally the immune system works to protect the body from unwanted visitors (pathogens and allergens); however, if an individual is genetically predisposed to certain chronic skin conditions, the immune system can be triggered to produce responses that are not desired. For example, in psoriasis, T-cells are activated by the production of inflammatory mediators leading to hyperproliferation of skin cells and inflammation (see
Eczema).
