Dermatology is an area of medicine that focuses on health issues affecting the skin, hair, nails, and mucous membranes. The skin is the largest organ of the body. It is also the first line of defense against pathogens and injury, and it can be a good indicator of overall health.It contains nerve endings, sweat glands, hair follicles, pores, blood vessels, and many other structures. Caring for it is important to your overall health.