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  • The skin participates directly in thermal, electrolyte, hormonal, metabolic, and immune regulation.
  • Percutaneous absorption depends on the xenobiotic's hydrophobicity, which affects its ability to partition into epidermal lipid, and rate of diffusion through this barrier.
  • The cells of the epidermis and pilosebaceous units express biotransformation enzymes.
  • Irritant dermatitis is a nonimmune-related response caused by the direct action of an agent on the skin.
  • Allergic contact dermatitis represents a delayed (type IV) hypersensitivity reaction, whereby minute quantities of material elicit overt reactions.

The skin protects the body against external insults in order to maintain internal homeostasis. It participates directly in thermal, electrolyte, hormonal, metabolic, and immune regulation. Rather than merely repelling noxious physical agents, the skin may react to them with various defensive mechanisms that serve to prevent internal or widespread cutaneous damage. If an insult is severe or intense enough to overwhelm the protective function of the skin, acute or chronic injury becomes readily manifest. The specific presentation depends on a variety of intrinsic and extrinsic factors including body site, duration of exposure, and other environmental conditions (Table 19–1).

Table 19–1 Factors influencing cutaneous responses.

Skin Histology

The skin consists of two major components: the outer epidermis and the underlying dermis, which are separated by a basement membrane (Figure 19–1). The junction ordinarily is not flat but has an undulating appearance (rete ridges). In addition, epidermal appendages (hair follicles, sebaceous glands, and eccrine glands) span the epidermis and are embedded in the dermis. In thickness, the dermis makes up approximately 90 percent of the skin and has largely a supportive function. Separating the dermis from underlying tissues is a layer of adipocytes, whose accumulation of fat has a cushioning action. The blood supply to the epidermis originates in the capillaries located in the rete ridges at the dermal–epidermal junction. Capillaries also supply the bulbs of the hair follicles and the secretory cells of the eccrine (sweat) glands. The ducts from these glands carry a dilute salt solution to the surface of the skin, where its evaporation provides cooling.

Figure 19–1

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