Melanin
Melanin is the pigment that gives skin, hair and eyes their colour. Made by melanocytes and passed to surrounding cells, it absorbs ultraviolet radiation, positioning itself over the cell nucleus where it shields the genetic material inside.
Melanin is a pigment with a function rather than a decoration. It absorbs ultraviolet radiation, and it does so from a specific position: carried up through the epidermis by keratinocytes, it gathers over the cell nucleus, an arrangement that has been described as holding a parasol above the genetic material.
It comes in more than one form, and the forms differ in how effectively they absorb. This is why skin tone reflects more than simple quantity: the type present matters alongside how much of it there is, and the number of melanocytes producing it varies surprisingly little between people.
Production responds to exposure. Ultraviolet radiation prompts melanocytes to make more, which is what tanning consists of, and it is also why a tan is described as a record of exposure that has already happened rather than a protection acquired in advance.
The word runs through a large part of dermatology. Vitiligo involves melanin no longer being produced in patches. Melasma involves it being produced in excess in a particular pattern. Moles are clusters of the cells that make it, and melanoma is a cancer arising from them.
Its protective effect is real and partial. More of it means longer before a given amount of radiation reaches vulnerable structures, and that is not equivalent to shade, clothing or sunscreen.
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