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Acne happens when oil (sebaceous) glands come to life around puberty, when these glands are stimulated by male hormones that are produced in the adrenal glands of both boys and girls.

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The oil glands, which are located just beneath the skin, continuously produce and secrete oil through openings in the skin. The oil lubricates and protects the skin. Under certain circumstances, cells that are close to the openings of the oil glands block the openings. This causes a buildup of oil underneath the skin.  Bacteria, which live in everyone's skin but generally mind their own business, feast on this oil, multiply, and cause the surrounding tissues to become inflamed if the follicle ruptures. 

 

If the inflammation is right near the surface, you get a pustule; if it's deeper, a papule (pimple); deeper still and it's a cyst.

 

If the pore of the affected hair follicle is closed to the surface, the result is a "whitehead." If the pore is wide open, oil becomes oxidized (that is, acted on by oxygen in the air), the oil changes from white to black, and the result is a "blackhead."