File:FDA -- Follicle3.jpg

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  Acne begins in pilosebaceous units, which consist of sebaceous glands and a single hair follicle. The sebaceous glands produce an oily substance called sebum.
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  Sometimes, hormones produced in the adrenal glands trigger the production of excess sebum, clogging the follicle and leading to bacterial growth. The resulting plug can appear dark and is called a blackhead, or open comedo.
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  If the plug is below the skin surface, it is called a whitehead, or closed comedo.
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  But, if the pocket of sebum and bacteria becomes too large, the follicle will rupture, spilling its contents into the surrounding tissue. These tissues are damaged when the body's immune system attempts a repair, resulting in a papule, pustule, cyst or abscess.

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