The skin is the largest organ of your body. It is part of the integumentary system which, along with skin, is composed of hair follicles, melanocytes, nerves, fat, blood vessels, and lymph vessels. The integumentary system’s most important function is protection. It protects your insides from the outside. Melanin is a barrier against UV rays, and skin is waterproof, so you can go for a swim without swelling up like a grape. The integumentary layer is composed of three parts, from outermost to innermost: the epidermis, the dermis, and the hypodermis. The epidermis is mainly composed of epithelial cells. The dermis contains connective tissues, blood vessels, lymphatic vessels, immune cells, nerves, and hair follicles. The hypodermis contains subcutaneous fat.