Acne usually appears in pre-teens and teens because the onset of puberty sets off a class of hormones called androgens. Androgens tend to stimulate the production of sebum, or skin oil which can build up and clog your pores. Oil trapped inside your pores becomes a breeding ground for bacteria. Your immune system's effort to fight off infection causes the redness, swelling and puslike fluid associated with acne.

Testosterone

Testosterone is a male hormone, but women also produce it at levels about one-tenth those of a man. In both men and women, testosterone stimulates the production of sebum, but since boys have a greater quantity of it, they are more likely to suffer from acne and to have a more severe form of it. Acne can be difficult to treat in teens because their hormone levels are in constant flux.

Menstruation

Women sometimes experience an increase in acne eruptions in the week preceding their menstrual period. Researchers are not exactly sure why, but they agree that it is related to hormonal changes during the cycle. Doctors sometimes prescribe birth control pills as a treatment because these drugs block androgens and lower testosterone.

Polycystic Ovary Syndrome

Polycystic Ovary Syndrome (PCOS) is a female reproductive disorder characterized by multiple small cysts in the ovaries. Women with this condition have high levels of androgens, including testosterone. These hormonal imbalances frequently cause acne, though not all women with acne have PCOS.

DHEAS

As boys and girls approach puberty, their adrenal glands begin to produce dihydroepiandrosterone sulfate (DHEAS), an androgen that is a building block for both testosterone and estrogen. DHEAS stimulates the production of sebum, which can lead to acne.

DHT

Di-Hydrotestosterone is another hormone that begins to appear at the onset of puberty. The body converts testosterone into DHT by a process that uses the enzyme 5 Alpha Reductase. The skin's oil glands have androgen receptors that attract DHT, which in turn stimulates oil production that can lead to acne.