The skin is the largest organ of the body. 

The skin and its derivatives (hair, nails, sweat and oil glands) make up the integumentary system. One of the main functions of the skin is protection. 

It protects the body from external factors such as bacteria, chemicals, and temperature. The skin contains secretions that can kill bacteria and the pigment melanin provides a chemical pigment defense against ultraviolet light that can damage skin cells.

Another important function of the skin is body temperature regulation. 

When the skin is exposed to a cold temperature, the blood vessels in the dermis constrict. This allows the blood which is warm, to bypass the skin. The skin then becomes the temperature of the cold it is exposed to.

Body heat is conserved since the blood vessels are not diverting heat to the skin anymore. Among its many functions the skin is an incredible organ always protecting the body from external agents.