The skin provides an overlaying protective barrier from the environment and pathogens while contributing to the adaptive immune system.

The skin helps protect our body’s internal structures from physical, chemical, biological, radiological, and thermal damage as well as damage from starvation and malnutrition. The skin is composed of tough skin cells as well as a tough protein called keratin that guard tissues, organs, and structures underneath the skin against physical damage from minor cuts, scratches, and abrasions. Because our skin is tough and largely waterproof, it helps protect internal structures from chemical irritants such as man-made detergents or even natural irritants like poison ivy. The waterproof nature of our skin also ensures that important molecules stay within our body.

The skin also contains important cells called Langerhans cells. These cells help our immune system fight off infectious biological agents, like bacteria that try to get further into our body through skin that may have been compromised by physical damage.

Sebaceous glands associated with the skin secrete substances that help fight off potentially dangerous microorganisms as well. These glands also secrete substances that help keep our skin hydrated, and thus more resistant to bacterial invasion.

Our skin also contains melanocytes that produce a pigment called melanin. This protects the body from radiological damage via the sun’s UV radiation (or that from tanning beds).

Part of our skin is made up of fat. This fat serves three large purposes:

1. It helps cushion internal structures against any physical blows.
2. It acts as a food source, protecting our body from the effects of starvation.
3. It helps insulate us against cold temperatures.

Our skin is also closely associated with sweat glands that help protect us from high temperatures by cooling us off through the process of evaporation. These glands also help to excrete potentially dangerous substances, like urea, out of the body.

All sorts of sensory receptors are found within the skin as well. These help move our body parts away from potential sources of damage, like hot stoves, when they sense danger, thereby protecting our body from great harm.

Finally, the skin is also important for the synthesis of vitamin D, which is an important vitamin for the building of strong and healthy bones. The skin protects the body from fractures if we do not otherwise get enough of this vitamin from food-based sources.

When you wear shoes that do not fit well and are a constant source of abrasion on your toes, you tend to form a callus at the point of contact. This occurs because the basal stem cells in the stratum basale are triggered to divide more often to increase the thickness of the skin at the point of abrasion to protect the rest of the body from further damage. This is an example of a minor or local injury, and the skin manages to react and treat the problem independent of the rest of the body. Calluses can also form on your fingers if they are subject to constant mechanical stress, such as long periods of writing, playing string instruments, or video games.

Hair is part of the integumentary system and serves a variety of functions. For example, hair on the head protects it from the sun and from heat loss; and hair in the nose and ears and around the eyes (eyelashes) defends the body by trapping dust particles that may contain allergens and microbes. Hair on the eyebrows prevents sweat and other particles from bothering the eyes. Hair also has a sensory function due to innervation of the hair papilla. Hair is extremely sensitive to changes in the environment, much more so than the skin surface. The hair root is connected to smooth muscles called arrector pili that contract in response to stimuli, making the external hair shaft “stand up.” This is visible in humans as goosebumps and even more obvious in animals, such as when a frightened cat’s fur puffs out. In many animals, the puffing out makes the animal appear larger, and could possibly enable it to scare off a predator. Another advantage of this ability to cause the hair to stand up is that it can trap air and can act as an insulator, decreasing heat loss.

The nail is a specialized structure of the epidermis that occurs at the tips of our fingers and toes. The nail body is formed on the nail bed, and it is designed to protect the tips of our fingers and toes, as they are the farthest extremities and the parts of the body that experience the maximum mechanical stress. The epidermis in this part of the body has evolved a specialized structure upon which nails can form. The nail body forms at the nail root. Lateral nail folds, folds of skin that overlap the nail on its side, help anchor the nail body. The nail fold that meets the proximal end of the nail body forms the nail cuticle, also called the eponychium. The nail bed is rich in blood vessels, making it appear pink, except at the base, where there is a crescent-shaped region called the lunula. The nail body is composed of keratin-rich, densely packed dead keratinocytes. The area beneath the free edge of the nail, where debris gets lodged, is called the hyponychium.