what causes acne?

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All acne is a disorder of the pilosebaceous unit, which is made up of a hair follicle, sebaceous gland, and a hair. These units are found everywhere on the body except on the palms, soles, top of the feet, and the lower lip. The number of pilosebaceous units is greatest on the face, upper neck, and chest. Sebaceous glands produce a substance called sebum, which is responsible for keeping the skin and hair moisturized. During adolescence sebaceous glands enlarge and produce more sebum under the influence of hormones, also called androgens. After about age 20, sebum production begins to decrease.Acne & BacteriaA 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 acne. The presence of bacteria attracts white blood cells to the follicle. These white blood cells produce an enzyme that damages the wall of the follicle, allowing the contents of the follicle to enter the dermis. This process causes an inflammatory response seen as papules (red bumps), pustules, and nodules. The bacteria also cause the formation of free fatty acids, which are irritants, increasing the inflammatory process in the follicle.A normal follicle looks like this:For reasons no one completely understands, follicles, often called pores, sometimes get blocked. Sebum (oil) which normally drains to the surface gets blocked and bacteria begins to grow. Both whiteheads and blackheads start out as a microcomedone. The picture below is a microcomedone:Normal FolliclesSebum produced by the sebaceous gland combines with cells being sloughed off 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.Obstructed FolliclesProblems arise when the sebum is trapped in the hair follicle. For reasons that are still unclear, some hair follicles become obstructed. The sebum is produced but gets trapped on the way out, and the cells that are normally sloughed off become "sticky", plugging up the follicle. The process of obstructing follicles is called comedogenesis. It causes some follicles to form a type of acne called comedones, also known as blackheads and whiteheads.Whitehead When the trapped sebum and bacteria stay below the skin surface, a whitehead is formed. Whiteheads may show up as tiny white spots, or they may be so small that they are invisible to the naked eye. BlackheadA blackhead occurs when the pore opens to the surface, and the sebum, which contains the skin pigment melanin, oxidizes and turns a brown/black color. It is not dirt and can not be washed away. Blackheads can last for a long time because the contents very slowly drain to the surface.A blackhead or whitehead can release its contents to the surface and heal. Or, the follicle wall can rupture and inflammatory acne can ensue (see below). This rupture can be caused by random occurrence or by picking or touching the skin. This is why it is important to leave acne prone skin relatively untouched.PapuleA papule occurs when there is a break in the follicular wall. White blood cells rush in and the pore becomes inflamed.PustuleA pustule forms several days later when white blood cells make their way to the surface of the skin. This is what people usually refer to as a "zit" or a "pimple".An inflamed lesion can sometimes completely collapse or explode, severely inflaming the surrounding skin, and sometimes engulfing neighboring follicles. These lesions are called nodules or cysts:Nodule When a follicle breaks along the bottom, total collapse can occur, causing a large, inflamed bump that can be sore to the touch.

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