What Is Skin?

The skin is the largest organ in the body, covering 1.8 square meters of surface area. Over that area the skin senses temperature, pressure, and pain; conserves water; sweats to cool the body; and heals itself if injured or infected. While many diseases such as acne or eczema primarily affect the skin, skin can also change because of diseases that affect other body systems. Before many of the present diagnostic tests were developed, physicians looked at the skin for information about the health of the entire body .

In addition to protecting, preserving water, and sensing for the body, the skin is presented to the world. Composed of the skin, hair, nails, and mucous membranes, the skin is a tremendously important part of how we see ourselves and how others view us. In a social environment, the skin is a large part of the interface between ourselves and the world. The skin gives signals about youth or age, and, along with the muscles of the face, expresses feelings. Because visual input is a significant part of communication, the skin’s appearance can have a large impact on how people interact with each other. The skin is organized into layers, and each level has a different function.

The most superficial upper layer is called the  epidermis, which contains  cells that constitute the uppermost skin layer and cells that make pigment. This self-regenerating layer develops into the top layer of “skin cells” and the pigment that gives the skin its color. Pigment cells, or  melanocytes, reside in the bottom layer of the epidermis, and when they are grouped together can appear as moles. The function of the epidermis is to be the front line of protection against water loss, physical stress, and  ultraviolet (UV ) radiation. The epidermis is especially thick on the palms and the soles of the feet and is often thickened in skin affected by psoriasis. Deeper in the skin, the dermis contains the skin’s blood vessels, nerves, hair follicles, sweat glands, and immune cells.

The dermis provides nutrients to support the epidermis and nerves to sense physical contact and trauma. When a cut, scrape, burn, or crack breaks through the epidermis, the nerves, immune cells, and supportive parts of the dermis sense this break and cooperate to repair it. Blood vessels bring nutrients that feed the skin, and immune cells both fight infection and, in the case of psoriasis, cause skin inflammation.Although treating skin and the diseases that affect it can be particularly challenging, the treatment options are exceptionally varied. Unlike most organs, skin can be treated with locally acting topical medicines such as creams or ointments that minimize systemic effects.