Definition Acne is a skin disorder resulting from the blockage of hair follicles in the skin. Excess cells produced in the follicle combine with sebum, an oily substance secreted by glands connected to the follicle. When a plug forms in the follicle and is infected with bacteria, a pimple erupts. Description Acne is a skin problem commonly associated with teenagers, but it can also occur in children, adults in their thirties or forties, and women around the time of menopause. It is most likely to appear on the parts of the body with the largest number of hair follicles upper back, and (in some people) upper arms. Some forms of acne are relatively mild. The less severe form consists of comedones, which are hair follicles blocked by plugs of sebum. If the comedo is open to the air on the skin surface, the sebum grows darker, giving the comedo a blackish appearance. This type of comedo is called a blackhead; its dark color is not caused by dirt. If the comedo is not open, the sebum insi...