Skin is one of the most vulnerable organs of the body. Skin is the largest organ of the integumentary system. Skin also helps maintain a constant body temperature. Skin is composed of three primary layers: the epidermis, which provides waterproofing and serves as a barrier to infection; the dermis, which serves as a location for the appendages of skin; and the hypodermis (subcutaneous adipose layer), which is called the basement membrane. Skin has pigmentation, or melanin, provided by melanocytes, which absorb some of the potentially dangerous ultraviolet radiation in sunlight. It also contains DNA repair enzymes which help to reverse UV damage, and people who lack the genes for these enzymes suffer high rates of skin cancer. One form predominantly produced by UV light, malignant melanoma, is particularly invasive, causing it to spread quickly, and can often be deadly. Human skin pigmentation varies among populations in a striking manner. This has sometimes led to the classification of people on the basis of skin color. Epidermis is the outermost layer of the skin. It forms the waterproof, protective wrap over the body’s surface and is made up of stratified squamous epithelium with an underlying basal lamina. The epidermis contains no blood vessels, and is nourished by diffusion from the dermis. The main type of cells which make up the epidermis are keratinocytes, melanocytes, Langerhans cells and Merkels cells. The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The skin provides a remarkably good barrier against bacterial infections. Skin disorders cover a wide range of conditions, some benign, some very serious, and some even a sign of another underlying illness. Skin disorders can be uncomfortable and may cause chronic disabilities. In addition, because the skin is so visible, skin disorders can lead to psychological stress. Many types of bacteria can infect the skin. The most common are Staphylococcus and Streptococcus. Skin infections caused by less common bacteria may develop in hospitals or nursing homes or while gardening or swimming in a pond, lake, or ocean. Some people are at particular risk of contracting skin infections?for example, people with diabetes because they are likely to have poor blood flow, especially to the hands and feet, and because high levels of sugar in the blood decrease the ability of white blood cells to fight infections. There are more than 2,000 known skin disorders. There are many disorders of the skin ,such as acne, Acrochordons ,Albinism , Alopecia (Baldness) ,Athlete’s Foot ,Behcet Disease ,Blisters ,Dry Skin (Xerosis) ,Ectodermal Dysplasia ,Eczma (Atopic Dermatitis) ,Ehlers-Danlos Syndrome and Itching. People with AIDS or other immune disorders and those undergoing chemotherapy are at higher risk as well, because they have a weakened immune system. Skin that is inflamed or damaged by sunburn, scratching, or other trauma is more likely to be infected. In fact, any break in the skin predisposes a person to infection. Prevention involves keeping the skin undamaged and clean.