Comedo: The primary sign of acne, consisting of a widened hair follicle filled with keratin skin debris, bacteria, and sebum (oil). A comedo may be closed or open. A closed comedo (called a whitehead) has an obstructed opening to the skin and may rupture to cause a low-grade inflammatory skin reaction in the area. An open comedo (called a blackhead) has a wide opening to the skin and is capped with a blackened mass of skin debris.
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