Tanning

Tanning is not the sun dyeing your skin. It is your skin reacting. When ultraviolet radiation hits the skin, melanocytes, the cells that make pigment, ramp up production of a protein called proopiomelanocortin, or POMC.

That protein is a precursor, a long molecule that is later cleaved into several active peptides. One of them is alpha melanocyte stimulating hormone. Those peptides bind to an antenna on the surface of the melanocyte, the melanocortin 1 receptor, and that raises production of eumelanin, the brown or black pigment, increases the formation of melanosomes, the little packets that hold the pigment, and steps up the handoff of those packets to keratinocytes, the cells that make up the bulk of the skin.

All of that increases the skin's photoprotection. It is like speeding up a factory that was already running. But the same radiation that triggers tanning also damages DNA: UVB forms thymine dimers, faulty bonds between two neighboring letters of the code, and UVA oxidizes it.

And not all skin responds the same way. Skin type I, very fair skin, burns easily and does not tan.