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Comedones are the skin-coloured, small bumps (papules) frequently found on the forehead and chin of those with acne.

- Open comedones are blackheads; black because of surface pigment (melanin) rather than dirt

- Closed comedones are whiteheads; the follicle is completely blocked

- Macrocomedones are facial closed comedones that are larger than 2-3 mm in diameter

-Solar comedones are found on the cheeks and chin of older people, and are thought to be due to sun damage.

- Larger and deeper uninflamed bumps are called nodules. They are more common on the trunk than on the face

The cells lining the sebaceous duct proliferate excessively in acne (cornification) and may block the sebaceous duct forming a comedone. These may be so small that they are not visible to the naked eye (microcomedones).

Comedones are aggravated by:

- Reduced levels of linoleate (a component of sebum) resulting in more scale and reduced barrier function

- Free fatty acids (made from sebum by bacteria)

- Excessive activity of the local hormone 5-testosterone (DHT)

- Interleukin 1 (IL-1) this is a chemical messenger (cytokine) produced by the cells lining the follicle

- Rupture of the follicle by injury eg by excessive squeezing of pimples, after abrasive washing, chemical peels or laser resurfacing, or simply from the presence of hard dried-up sebum poking through the wall of the follicle

- Overhydrated skin eg premenstrually, from moisturisers or in tropical conditions

- Contact with certain chemicals including isopropyl myristate, propylene glycol and some dyes (which may be found in certain cosmetics)

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