The skin is a protective tissue-layered stratum that encases the body and guards it from potentially harmful intruders. It is the body's first defense against pathogens like viruses and bacteria. The constituent components of the skin perform a second major skin function of equal importance, sensation. Without the use of tiny receptors and nerve endings in the dermis and epidermis, we would not be able to feel, interpret pain and pleasure, or position our bodies safely. Encapsulated and free nerve endings, corpuscles (e.g., Meissner's corpuscle, found mostly in hairless skin, is a mass of sensory/touch encapsulated nerve endings), and sensory receptors for touch and pressure all communicate tactile information to neurotransmitters.
Under normal circumstances, without the application of external stimulus, skin temperature and body temperature are the same or similar. The skin works to balance and maintain its temperature according to changes in the environment, heart rate, and the nervous system. Thermoreceptors respond to temperature and trigger our cutaneous sensation of cold and hot. The skin also responds to internal changes in temperature. Febrile disease often affects skin temperature, making it hot and red in some cases, and cold and clammy in others.
Sensible exposure to sunlight synthesizes the production of vitamin D through interaction with ergosterol, a naturally occurring fat found in the skin. Vitamin D absorption helps metabolize calcium and phosphorous, which is important to bone and tooth health.
Finally, "gooseflesh" or "goosebumps," called piloerection, is a skin reaction that occurs when the pilo muscles surrounding the follicles are activated by cold or shock. Under normal conditions, the pilo muscles keep hair somewhat erect. But cold and shock trigger them to become hyperactive, causing the hair to stand up straight. This sensation may be the first reaction the body has to cold weather, and it often occurs before a person realizes he or she is cold. When it happens in the experience of shock or fright, it is usually sensed and observed on the back of the neck and on the arms.