The common term for the outer layer of skin is the epidermis. The epidermis serves as a protective barrier against environmental threats, such as bacteria, UV radiation, and chemicals. It is also involved in water retention and sensory perception. The epidermis is composed of layers of keratinocytes, which produce keratin, a protein that aids in waterproofing. Below the epidermis lies the dermis, which contains blood vessels, nerves, and connective tissue.