Acne is a common skin condition in which the hair follicles become clogged with sebum. The hair follicles are the openings around the hair shaft, and sebum is the oil produced by the glands within the follicle. The clogged follicles cause pimples and inflamed infected abscesses, or collections of pus.
Acne tends to develop in teenagers because of an interaction among hormones, sebum, and bacteria.
During puberty, the glands in the skin produce excessive sebum. In acne-prone skin, the sebum and dead skin cells clog the hair follicles and form comedones, or clogged pores. Acne may occur on a person's face, neck, chest, back, shoulders, scalp, and upper arms and legs. Comedones, or clogged pores, are the first signs of acne. This causes a pimple or pustule.