The skin, together with its accessory structures (hair, nails, glands, sensory receptors, muscles, and nerves), constitute what is known as the integumentary system. The skin itself is an organ composed of several tissues (epidermal, connective, nervous, and muscular) and is one of the largest organs of the body in terms of its surface area (approximately 1.75 m2) and weight (approximately 5 kg). It has a variety of functions, chief among which is protecting underlying tissues from microbes.

Anatomy and physiology of human skin

It is important to realise that the structure of skin is not uniform over the whole body surface, and profound differences occur at different body sites. Obvious examples are the presence or absence of hair and sudoriferous glands. Nevertheless, certain features are common to skin regardless of its location. Hence, it is basically composed of two layers – an inner dermis and an outer epithelium known as the epidermis (Figure 2.1). The epidermis is a keratinised, stratified, squamous epithelium within which five layers can be distinguished (Table 2.1 and Figure 2.2). Its thickness varies from 0.5 to 3 mm, depending on its location and, being the outermost layer of the skin, it is obviously an important site for microbial colonisation. The most common cell of the epidermis (comprising approximately 90% of all the cells of the skin) is the keratinocyte. New keratinocytes are constantly being produced in the stratum basale and, as they are pushed towards the surface, they undergo a process known as keratinisation.