Ultraviolet radiation increases the number of active melanocytes, consequently stimulating the production of melanin.
This substance (also present in the hair and in some parts of the eye) is synthesized by melanocytes from tyrosine, tryptophan and phenylalanine. It is responsible of tanning in the short and long term. In the first case, usually only seen in people with moderately dark complexion, there is an oxidation of melanin, which tends to become darker.
These phenomenons manifest itself immediately after solar exposure and recede within a few minutes.
The melanin, while we ensure a darker color, the other protects the skin from the effects of solar radiation by acting as a real filter. This substance is able to impede the penetration of ultraviolet rays in the deeper layers of skin. More on How our skin is protects by erythematictic
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