Topical drugs (drugs applied directly to the skin) are a mainstay of treating skin disorders. Systemic drugs are taken by mouth or given by injection and are distributed throughout the body. Rarely, when a high concentration of a drug is needed at the affected area, a doctor injects the drug just under the skin (intradermal injection).
The skin provides a remarkably good barrier against bacterial infections. Although many bacteria come in contact with or reside on the skin, they are normally unable to establish an infection. When bacterial skin infections do occur, they can range in size from a tiny spot to the entire body surface. They can range in seriousness as well, from harmless to life threatening.
A blister (bulla, or, when small, a vesicle) is a bubble of fluid that forms beneath a thin layer of dead skin. The fluid is a mixture of water and proteins that oozes from injured tissue. Blisters most commonly form in response to a specific injury, such as a burn or irritation, and usually involve only the topmost layers of skin. These blisters heal quickly, usually without leaving a scar. Blisters that develop as part of a systemic (bodywide) disease may start in the deeper layers of the skin and cover widespread areas. These blisters heal more slowly and may leave scars.
Corns are hard cone-shaped bumps of skin commonly found on the top surface of the smaller toes, particularly over a joint. Calluses are broader, flat thickenings of the skin usually located on the palms or soles.
Fungi usually make their homes in moist areas of the body where skin surfaces meet: between the toes, in the genital area, and under the breasts. Common fungal skin infections are caused by yeasts (such as Candida or Malassezia furfur) or dermatophytes, such as Epidermophyton, Microsporum, and Trichophyton. Many such fungi live only in the topmost layer of the epidermis (stratum corneum) and do not penetrate deeper. Obese people are more likely to get these infections because they have excessive skinfolds, especially if the skin within a skinfold becomes irritated and broken down ( intertrigo). People with diabetes tend to be more susceptible to fungal infections as well.
Hair originates in the hair follicles. These follicles are located in the dermis, which is the skin layer between the epidermis (the surface layer) and the fat layer (also called the subcutaneous layer). Hair follicles are present everywhere on the skin except the lips, palms of the hands, and soles of the feet. New hair is made in the hair matrix at the base of the hair follicle. Living cells in the hair matrix multiply and push upward. These cells rapidly dehydrate, die, and compact into a dense, hard mass that forms the hair shaft. The hair shaft, which is made up of dead protein, is covered by a delicate covering (cuticle) composed of platelike scales.
The immune system plays a vital role in maintaining the health of all the tissues of the body. The immune system reacts to invaders, such as microorganisms, foreign substances, or cancer cells, and triggers inflammation to attack these invaders. Usually the immune system reaction protects the body and aids healing. However, sometimes the immune system overreacts, or the reaction is misdirected at healthy tissues and causes intense inflammation and damage. These abnormal immune system responses are called hypersensitivity reactions. Some hypersensitivity reactions are called allergies, especially when they occur after exposure to substances that are usually harmless to most people. Hypersensitivity reactions can involve the skin and cause disorders such as the following:
Many disorders can affect the nails, including deformity and dystrophy, injuries, infections, and ingrown toenails. Infections can involve any part of the nail and may or may not change the nail's appearance. Most nail infections are fungal ( onychomycosis), but bacterial and viral infections occur.
Cells of the skin and underlying tissue may accumulate and cause growths. Growths may be raised or flat and range in color from dark brown or black to skin-colored to red. They may be present at birth or develop later.
Melanin is the pigment that produces the various shades and colors of human skin, hair, and eyes. Coloration (pigmentation) is determined by the amount of melanin in the skin. Without melanin, the skin would be pale white with shades of pink caused by blood flow through the skin. Fair-skinned people produce very little melanin, darker-skinned people produce moderate amounts, and very dark-skinned people produce the most. People with albinism have little or no melanin and thus their skin appears white or pale pink. Usually, melanin is fairly evenly distributed in the skin, but sometimes people have spots or patches of skin with more melanin. Examples of such spots include freckles, age spots ( lentigines), and melasma.
Psoriasis, parapsoriasis, pityriasis rosea, pityriasis rubra pilaris, lichen planus, and lichen sclerosus are different skin disorders that have been grouped together because the bumps, rashes, scales, and skin discoloration they cause have similar characteristics. That is, the rashes and bumps have well-defined borders, and the scales usually do not crust, crack, or weep with fluid.
Skin cancer is the most common type of cancer. Skin cancer is most common among people who work or play sports outside and among sunbathers. Fair-skinned people are particularly susceptible to developing most forms of skin cancer because they produce less melanin. Melanin, the protective pigment in the outer layer of skin (epidermis), helps protect the skin from ultraviolet (UV) light. However, skin cancer also can develop in dark-skinned people and in people whose skin has not had significant sun exposure. Skin cancers may also develop years after x-ray therapy or exposure to substances that cause cancer (for example, ingestion of arsenic).