 It provides immunologic information obtained during antigen processing to the appropriate effector cells in the lymphatic tissue.
 It participates in homeostasis by regulating body temperature and water loss.  It conveys sensory information about the external environment to the nervous system.
 It performs endocrine functions by secreting hormones, cytokines, and growth factors and converting precursor molecules into hormonally active molecules (vitamin D). It functions in excretion through the exocrine secretion of sweat, sebaceous, and apocrine glands.
In addition, certain lipid-soluble substances may be absorbed through the skin. Although not a function of skin, this property is frequently used to deliver therapeutic agents. For example, nicotine, steroid hormones, and seasickness medications are frequently delivered through the skin in the form of small sticking plasters or patches. To reduce nicotine withdrawal symptoms during smoking cessation, nicotine patches are often used to provide a small constant dose of nicotine without the dangerous effects of tobacco smoke.
Skin is categorized as thick or thin, a reflection of thickness and location.
The thickness of the skin varies over the surface of the body, from less than 1 mm to more than 5 mm. However, the skin is obviously both grossly and histologically different at two locations: The palms of the hands and the soles of the feet. These areas are subject to the most abrasion, are hairless, and have a much thicker epidermal layer than skin in any other location. This hairless skin is referred to as thick skin. Elsewhere, the skin possesses a much thinner epidermis and is called thin skin. It contains hair follicles in all but a few locations.
The terms thick skin and thin skin, as used in histologic description, are misnomers and refer only to the thickness of the epidermal layer. Anatomically, the thickest skin is found on the upper portion of the back where the dermis is exceedingly thick. The epidermis of the upper back, however, is comparable to that of thin skin found elsewhere on the body. In contrast, in certain other sites such as the eyelid, the skin is extremely thin.
The epidermis is composed of stratified squamous epithelium in which four distinct layers can be identified. In the case of thick skin, a fifth layer is observed (Figs. 15.1 and 15.2). Beginning with the deepest layer, these are as follow:  the stratum basale, also called the stratum germina tivum because of the presence of mitotically active cells, the stem cells of the epidermis; the stratum spinosum, also called the spinous layer or prickle cell layer because of the characteristic light microscopic appearance of short processes extending from cell to cell; the stratum granulosum, which contains numerous in tensely staining granules; the stratum lucidum, which is limited to thick skin and considered a subdivision of the stratum corneum; and the stratum corneum, which is composed of keratinized cells.
FIGURE 15.1 • Photomicrograph showing the layers of thin skin. This hematoxylin and eosin (H&E)–stained specimen from human skin shows the two chief layers of the skin—the epidermis (Epi) and dermis (Derm). The epidermis forms the surface; it consists of stratified squamous epithelium that is keratinized. The dermis consists of two layers: The papillary layer, which is the most superficial layer and is adjacent to the epidermis, and the more deeply positioned reticular layer. The boundary between these two layers is not conspicuous; the papillary layer is, however, more cellular than the reticular layer. In addition, the collagen fibers of the reticular layer are thick (clearly visible in the lower part of the figure); those of the papillary layer are thin. 45.
Differentiation of epithelial cells constitutes a specialized form of apoptosis.