The largest organ outside the body is the skin. It serves as a protective barrier against pathogens, helps regulate body temperature, and allows sensory perception through nerve endings. The skin is composed of three main layers: the epidermis, dermis, and hypodermis. In adults, the skin covers an area of about 1.5 to 2 square meters and plays crucial roles in immunity and overall health. It also has various functions, including vitamin D synthesis when exposed to sunlight, making it essential not just for protection but for bodily functions as well.