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How Does Acne Develop?

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Updated December 14, 2003

Doctors describe acne as a disease of the pilosebaceous units. Found over most of the body, pilosebaceous units consist of a sebaceous (oil) gland connected to a hair-containing canal called a follicle. These units are largest and most numerous on the face, upper back, and chest--areas where acne tends to occur. The sebaceous glands make an oily substance called sebum that normally empties onto the skin surface through the opening of the follicle.

Normal Pilosebaceous Unit

Acne is believed to result from a change in the inner lining of the follicle that prevents the sebum from passing through. For reasons not understood, cells from the lining of the follicle are shed too fast and clump together. The clumped cells plug up the follicle's opening so sebum cannot reach the surface of the skin. The mixture of oil and cells causes bacteria that normally live on the skin, called Propionibacterium acnes (P. acnes), to grow in the plugged follicles. These bacteria produce chemicals and enzymes that can cause inflammation. (Inflammation is a characteristic reaction of tissues to disease or injury and is marked by four signs: swelling, redness, heat, and pain.) When the plugged follicle can no longer hold its contents, it bursts and spills everything onto the nearby skin--sebum, shed skin cells, and bacteria. Lesions or pimples develop as a result of the skin's being irritated.

People with acne frequently have a variety of lesions. The basic acne lesion, called the comedo (kom-e-do) or comedone, is simply an enlarged hair follicle plugged with oil and bacteria. This lesion is often referred to as a microcomedo because it cannot be seen by the naked eye. If the plugged follicle, or comedo, stays beneath the skin, it is called a closed comedo or whitehead. Whiteheads usually appear on the skin surface as small, whitish bumps. A comedo that reaches the surface of the skin and opens up is called a blackhead because it looks black on the skin's surface. This black discoloration is not due to dirt. Both whiteheads and blackheads may stay in the skin for a long time.

Other troublesome acne lesions can develop, including the following:

  • Papules--inflamed lesions that usually appear as small, pink bumps on the skin and can be tender to the touch.

  • Pustules (pimples)--inflamed, pus-filled lesions that can be red at the base.

  • Nodules--large, painful, solid lesions that are lodged deep within the skin.

  • Cysts--deep, inflamed, pus-filled lesions that can cause pain and scarring.

Reprinted from The National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS)

Acne Resource Center

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