Skin cells are the basic building blocks of the skin, which is the largest organ in the human body. These cells form a protective barrier that helps keep out harmful substances and prevents water loss. The outermost layer of skin, called the epidermis, is primarily made up of a type of skin cell known as keratinocytes, which produce a protein called keratin that strengthens the skin. In addition to keratinocytes, the skin contains other important cell types, such as melanocytes, which produce the pigment melanin that gives skin its color and protects against UV radiation. Skin cells continuously regenerate, with new cells forming and old ones shedding, helping to maintain healthy skin throughout a person's life.