Benign moles are extremely common on human skin, but their functions (if any) are unknown. There are theories however - the most prominent being that the extra melanin produced in the moles' melanocytes helps to protect against UV radiation. But melanin-rich sites in human skin are often prevalent in areas where little or no sunlight ever reaches. Further, many nocturnal mammals - bats for example - have skin rich in melanocytes though they are not in any danger from UV in sunlight.
Alternative theories suggest that moles may play a role in immunological defence.
“ […] melanocytes are not simply pigment-producing cells, but produce substances with a range of biological functions, including structural strengthening by cross-linking proteins, antimicrobial defense, photon shielding, and chemoprotection. […] to provide several physiologically significant functions, including the provision of communicatory links with several different systems, e.g. the skin, central nervous system, and immune/ inflammatory responses.”
See: 'The mole theory: primary function of melanocytes and melanin may be antimicrobial defense and immunomodulation (not solar protection)' International Journal of Dermatology, 44, 340–342.
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