A shield against heat, light, injury and infection, the skin is the body's largest organ. Weighing about six pounds, the skin regulates body temperature and stores water, fat and vitamin D. The skin is made up of two main layers: the outer epidermis and the inner dermis.

The epidermis, which is the outer layer of the skin, is made up mostly of flat, scaly cells called squamous cells. Under the squamous cells lay round cells called basal cells. The deepest part of the epidermis also contains melanocytes. These cells produce melanin, which gives skin its color.

The dermis contains blood vessels, lymph vessels, hair follicles and glands. Some of these glands produce sweat, which helps regulate body temperature. Other glands produce sebum, an oily substance that helps prevent the skin from drying out. Sweat and sebum travel to the skin's surface through small openings called pores.