The average adult has over 8 lbs. of skin, and it is the largest organ in your body. Your skin serves many purposes, but one of its major functions is to protect the body from infectious organisms, such as parasites, bacteria or viruses, that cause disease. Other ways skin protects you from disease include its role in alerting the immune system to the presence of harmful organisms, producing and excreting antibacterial substances, and supporting the growth of "healthy" bacteria.
The outermost layer of skin, called the epidermis, acts as a physical barrier between your body and the dirt and bacteria outside. Each cell in your epidermis, called a keratinocyte, is tightly connected to its neighbors, so that no bacteria can pass between them. As long as your skin is intact and healthy, bacteria and viruses cannot penetrate the barrier. If your skin is injured, such as from a blister, a splinter, a cut or a burn, bacteria can enter your body and cause an infection.
The inner layer of your skin, called the dermis, contains several different types of cells that protect you from disease. To help keep your skin moist and healthy, sebaceous glands located in the dermis produce sebum. Sebum is an oily or waxy substance that helps your skin retain moisture and prevent drying out. The skin must remain moist to stay healthy; if your skin becomes to dry, cracks can form that allow bacteria to enter your body. Sebum also protects you from harmful bacteria by making your skin slightly acidic, creating an environment in which some harmful bacteria cannot survive.
Your skin also contains Langerhans cells. These Langerhans cells are a very important part of your immune system; if they sense that harmful bacteria has invaded the skin, they release a chemical signal that attracts white blood cells to that area of your body to fight off the infection. Your skin becomes red and swollen as the chemicals released by your skin's Langerhans cells attract many other immune cells to fight the infection and extra blood to nourish the injured tissues.
Millions and millions of healthy bacteria, called normal flora, colonize the outside of your skin. These bacteria live on your skin and use dead skin cells for their energy. Though normal flora bacteria are not usually harmful, they can cause infections if they enter your body through a wound. Most of these organisms are considered "healthy" bacteria because they prevent very harmful bacteria, called pathogens, from growing on your skin. As long as your body is colonized by healthy bacteria, there is very little room for the pathogens to grow.
If you should happen to injure your skin, you must keep the area clean until it heals and the protective functions of the skin are restored. Wash minor cuts and injuries with soap and warm water to remove dirt and bacteria, then use an antibiotic ointment and cover the wound with an adhesive bandage to protect the injury while the skin heals.