IMAGINE FOR A MOMENT WHAT THE HUMAN BODY WOULD BE LIKE WITHOUT SKIN (we know—ick-but bear with us): All our organs, muscles, and bones would be exposed, completely vulnerable to the ravages of the environment. Instead, we are covered from head to toe with a powerful shield that not only protects us but also does other pretty amazing things.

"The skin has many vital functions, but one of the most important is preventing the loss of water from the body to stave off dehydration," says UPMC dermatologist Laura Ferris, M.D., Ph.D., a professor at the University of Pittsburgh Clinical and Translational Science Institute. Rather than just being the equivalent of a giant tarp wrapped around our skeleton, however, the skin is actually the body's largest organ (an average of 22 square feet, weighing about eight pounds!), with multiple intricate processes occurring inside its cells. "The skin has its own active and complex immune system that prevents you from becoming infected with bacteria and viruses we come in contact with every day," Dr. Ferris says. Other important functions this multitalented organ performs: protecting you from deeper injury, regulating body temperature, being your first line of defense against environmental toxins and the sun's ultraviolet rays, and warning you when danger lurks (those little hairs that stand up when you're anxious or afraid are controlled by tiny muscles in the skin).