Melanin pigment, or simply melanin, is a compound of dark brown to black pigmentation that can be found in several parts of the body of humans and animals. It is most commonly associated with skin color, although the eyes and hair also contain melanin. It is produced by a class of pigment-producing cells called melanocytes, which are located in the bottom layer of the skin's outer layer, known as the epidermis. When melanocytes create melanin pigment, a process called melanogenesis, it creates a color that is permanent.
As the main determinant of skin pigmentation, melanin pigment indicates greater concentration with darker skin and lesser concentration with lighter skin. People who have limited or absolutely no melanin develop a condition called albinism. Everyone, regardless of race or ethnicity, has more or less the same number of melanocytes. Melanin pigment is considered a derivative of tyrosine, which is an amino acid that cells use for the synthesis of proteins.
The two major types of melanin pigment are eumelanin and pheomelanin. The more common of the two, eumelanin, is the pigment that is found in darker-skinned people. It is also responsible for coloring the hair black, brown, grey and yellow, as well as the areola, which is the darker circular area surrounding each nipple of the breast. Pheomelanin is more closely associated with fairer-skinned people. It is responsible for giving hair a color that roughly ranges from red to yellow, which is why it is more common with red-haired individuals.
Besides melanogenesis, melanin pigment can be produced by DNA damage from ultraviolet (UV) radiation. This process is most commonly induced by tanning, which involves darkening the skin, usually by the sun's ultraviolet rays or tanning booths. This type of melanin production, however, unlike melanogenesis, does not cause permanent pigmentation.
Melanin pigment acts as a photoprotectant, which means that it lessens the amount of harm that UV radiation does to the skin. Eumelanin is a much better photoprotectant than pheomelanin, which has a higher risk of becoming a carcinogen, or a skin cancer-causing agent. Higher concentrations of melanin, though, while limiting exposure to sunlight, deprives the skin of getting the agent needed to produce vitamin D.
The same logic applies to the eyes. The melanin that colors the iris—that thin, circular structure that regulates the amount of light going to the retina—protects it from the sun's potentially harmful rays. People with lighter-colored irises are at larger risk.