What exactly is acne?

Acne is a disorder of the pilosebaceous unit. The pilosebaceous unit is made up of the sebaceous gland, a hair follicle, and a hair. These units are found everywhere on the body except on the palms, soles, the lower lip, and the top of the feet. The number of pilosebaceous units is greatest on the face, upper neck, and chest. Sebaceous glands produce sebum, a substance responsible for moisturizing the hair and skin. During adolescence sebaceous glands enlarge and produce more sebum under the influence of hormones. These hormones are called androgens. Sebum production usually starts decreasing after about age 20, although sometimes is can continue into adult years.

A bacteria, known as Propionibacterium acnes, is a normal inhabitant of the skin. It uses sebum as a nutrient for growth, therefore increases in follicles during puberty. People with acne have more Propionibacterium acnes in their follicles than people without. The presence of the bacteria attracts white blood cells towards the follicle. These white blood cells produce an enzyme that causes damage to the wall of the follicle, allowing the contents of the follicle to enter the dermis (a layer of skin). This process causes an inflammatory response seen as papules (red bumps), pustules (a small collection of pus in the top layer of skin), and nodules (small aggregation of cells). The bacteria also cause the formation of free fatty acids, which increase the inflammatory process in the follicle.

Sebum produced by the sebaceous gland combines with cells being discarded within the hair follicle and "fills up" the hair follicle. When the follicle is "full", the sebum spreads over the skin surface giving the skin an oily appearance. When this process works correctly, the skin is moisturized and remains healthy.

Problems arise when the sebum is trapped inside the hair follicle. Some hair follicles become obstructed, for reasons that are still not clear. The sebum is produced but gets trapped on the way out, and the cells that are normally discarded become "sticky", plugging up the follicle. This process is called comedogenesis. It causes some follicles to form a type of acne called comedones, more commonly known as blackheads and whiteheads.