The skin, we are often told, is the body's largest organ. On most adults it measures two square yards and weighs six to nine pounds. 

But what's surprising is not the size but the notion that skin is an organ. It seems to be in a category different from the heart, the stomach, and the lungs -- more like clothing, something meant simply to hold in the important stuff and make us look good. Indeed, skin serves both of those vital purposes.

But skin does crucial physiological work. It protects the body from dirt, chemicals, and microorganisms and functions as a host to beneficial bacteria. It blocks the sun's ultraviolet (UV) rays even as it soaks in enough to produce the vitamin D we need to absorb calcium and build strong bones. And skin's pores open and close to keep our temperature steady. 

Humans' ability to shed heat by sweating enabled the evolution of a large brain, which could not operate without a mechanism for staying cool. "We have many more sweat glands per unit area than other mammals," says Nina Jablonski, head of the anthropology department at Penn State University.

Skin accommodates every stretch and twist we make, expanding for pregnancy or obesity. It renews its topmost layer, the epidermis, every month or so, the better to heal quickly from most cuts and scratches and maintain a fresh line of defense against the outside world.

And skin serves as a conduit for the network of nerves that yields our sense of touch. Of the five senses, touch is the least noted because its influences on human well-being are more subtle," Jablonski writes in "Skin: A Natural History" (University of California; 2006). Perhaps that subtlety arises from the number of ways we use touch -- to feel the warmth of the sun or the strength of a breeze, to assess the quality of garden soil or the ripeness of a tomato, to comfort and care for children. We know from studies that when we exchange caring touches with other people, our levels of stress hormones and anxiety are reduced, Jablonski says. And when we receive touch -- a hug or a massage -- we feel a sense of relief."

Our skin does so much, it's easy to forget it does anything at all -- until it begins to itch, hurt, or, heaven help us, look ugly. Avoiding these calamities altogether would be impossible. But by understanding what can go wrong, we can keep the most troublesome problems to a minimum.

Rashes 
Americans make some 25 million visits a year to dermatologists, and 10 million of these are for help with a rash. Most occur when the immune system goes to work against a perceived threat: a poisonous plant, an insect bite, an irritating chemical, fungal spores, or bacteria. In cases of chronic rash, the offender can be hard to pin down.

Eczema, for example, produces dry, itchy, and sometimes oozy patches, typically on the face, neck, elbows, knees, and ankles. Triggers include dust mites and dander. Rough fabric, heat, and soaps can exacerbate symptoms.

The thick, scaly patches that characterize psoriasis, most commonly seen on the elbows, knees, and lower back, seem to be prompted by medicines, stress, or injury to the skin. An autoimmune disease, psoriasis occurs when the system mistakenly speeds up the production of skin cells.

And the itchy, flaky skin of seborrheic dermatitis, which forms in the eyebrows, in the creases of the nose, behind the ears, and on the scalp (where it creates dandruff), happens when the immune system attacks fungal spores that live in hair follicles. The skin usually tolerates these spores peacefully, but cold weather conditions, infrequent shampooing, or stress can cause an outbreak.

All these conditions -- eczema, psoriasis, and seborrhea -- run in families and ebb and flow with stress and, for women, the hormonal fluctuations that occur with pregnancy, menstrual cycles, and menopause, says Diane Berson, a dermatologist in New York City. For these three chronic rashes, flare-ups can be prevented by using a good moisturizer. Doctors can also prescribe anti-inflammatory treatments and, for seborrhea, antifungal creams.