SKINPIGMENTS

INTRODUCTION

Various shades and colors of human skin are created by the brown pigment, melanin. Without melanin, the skin would be pale white with varying shades of pink caused by the blood flowing through it. Fair-skinned people produce very little melanin; darker-skinned people produce moderate amounts; and very dark skinned people produce a great deal. People with albinism have no melanin.
Melanin is produced by special cells (melanocytes) that are interspersed among the other cells in the top layer of the skin, the epidermis. After melanin is produced, it spreads into other nearby skin cells.
When exposed to sunlight, melanocytes produce increased amounts of melanin, causing the skin to darken, or tan. In some fair-skinned people, certain melanocytes produce more melanin than others in response to sunlight. This uneven melanin production results in spots of pigmentation known as freckles. A tendency to freckles runs in families. Increased amounts of melanin can also occur in response to hormonal changes, such as those that may take place in Addison's disease, in pregnancy, or with oral contraceptive use. Some cases of skin darkening, however, are not related to increased melanin at all, but rather to abnormal pigments that make their way into the skin. Diseases such as hemochromatosis or hemosiderosis or some drugs that are applied to the skin, swallowed, or injected can cause skin darkening. A buildup of bilirubin (the main pigment in bile) causes the skin to turn yellow (jaundice).
An abnormally low amount of melanin (hypopigmentation) may affect large areas of the body or small patches. Decreased melanin usually results from a previous injury to the skin such as a blister, ulcer, burn, or skin infection. Sometimes pigment loss results from an inflammatory condition of the skin or, in rare instances, is hereditary. A common skin infection, tinea versicolor BACTIRIAL SKIN DISORDERS

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.
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 people while hospitalized or living in a nursing home, while gardening, or while swimming in a pond, lake, or ocean.
Some people are at particular risk of contracting skin infections. For example, people with diabetes are likely to have poor blood flow, especially to the hands and feet, and the high levels of sugar in their blood decrease the ability of white blood cells to fight infections. People with human immunodeficiency virus (HIV) or 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. When the skin is cut or scraped, the injury should be washed with soap and water and covered with a sterile bandage. Antibiotic creams and ointments may be applied to open areas to keep the tissue moist and to try to prevent bacterial invasion. If an infection develops, small areas may be treated with antibiotic creams. Larger areas require antibiotics taken by mouth or given by injection. Abscesses (pus-filled pockets) should be cut open by the doctor and allowed to drain, and any dead tissue must be surgically removed.