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How acne developsDoctors describe acne as a disease of the pilosebaceous units (PSUs). Found over most of the body, PSUs consist of a sebaceous gland connected to a canal, called a follicle that contains a fine hair. These units are most numerous on the face, upper back, and chest, which are the typical areas for breakouts and problems. Cells called keratinocytes line the follicle. The hair, sebum, and keratinocytes that fill the narrow follicle may produce a plug, which is an early sign of acne. The plug prevents sebum from reaching the surface of the skin through a pore. The mixture of oil and cells allows bacteria Propionibacterium acnes (P. acnes) that normally live on the skin to grow in the plugged follicles. These bacteria produce chemicals and enzymes and attract white blood cells that 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 wall of the plugged follicle breaks down, it spills everything into the nearby skin leading to lesions or pimples. People with acne frequently have a variety of lesions, the basic acne lesion, called the comedo (KOM-e-do), is simply an enlarged and plugged hair follicle. If the plugged follicle, or comedo, stays beneath the skin, it is called a closed comedo and produces a white bump called a whitehead. A comedo that reaches the surface of the skin and opens up is called an open comedo or blackhead because it looks black on the skin's surface. This black discoloration is due to changes in sebum as it is exposed to air. It is not due to dirt. Both whiteheads and blackheads may stay in the skin for a long time Other acne lesions:
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Acne
and conditions
