Skin Forms a remarkable protective barrier against the outside world, helping to regulate temperature and fluid balance, keeping out harmful germs and chemicals, and offering natural protection against sunlight. It grows hair, sweats, produces oil for self-lubrication, and gives an accurate perception of touch, heat, cold, and pain, as well as the more complex sensations such as tickle, itch, and pressure.

Skin caliber varies in different body sites, from the thin, delicate tissue of the eyelid, to the thick, hard padding of the heel. Some sites have particular characteristics - luxuriant hair growth on the scalp, sweating in the armpits, highly-tuned touch discrimination in the fingertips - yet the basic structure of the skin is the same everywhere, although its actual thickness varies between different physical types. Redheads, for example, have considerably thinner skin than brunettes, and, consequently, their skin often reacts with more sensitivity.

The outer of the three skin layers, the epidermis, comprises the skin surface of dead "horny" cells, and underlying "prickle" cells, which move upward to replace them as they are shed (around 90 percent of household dust is dead skin cells). The epidermis also contains melanocytes, cells that secrete melanin, the pigment responsible for varying skin color. Melanin production is stimulated by sunlight: in white races this creates a tan, which offers some protection against further exposure to the sun's ultraviolet rays.

Under the epidermis is the middle layer, or dermis, a bulky fibrous and protein layer containing hair follicles, sweat, and sebaceous (oil-producing) glands, and specialized sensory receptors for conveying information about touch, temperature, and pain. Mast cells in the dermis respond to physical or chemical damage by releasing histamine, a substance leading to allergic responses.

Finally there is a layer of subcutaneous fatty tissue that gives the skin its feeling of softness and plumpness and acts as insulation against the cold. The blood vessels supplying nourishment to the outer skin layers are carried in this layer.