DERMPICS / SCIENCE PHOTO LIBRARY DERMPICS / SCIENCE PHOTO LIBRARY
Halo nevus next to a nevus without halo. A nevus is a birthmark, usually composed of clusters of blood vessels, and a halo nevus has a lighter area of skin around it. Nevi are caused by overactivity or abnormality of melanocytes (skin cells that produce the brown pigment melanin). Most nevi are harmless and do not require treatment.
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