Skin. It is the largest organ of the body. The first barrier to the outside. It keeps out many harmful bacteria and other things. Of course, it also keeps in all the things we need inside our bodies.

The skin helps control body temperature. Glands on the skin release fluid to cool the body when it gets too hot. When a person gets too cold, blood vessels in the skin narrow. This helps to trap heat inside the body.

Yet, like other organs of the body, the skin can have problems. Almost any teenager can tell you the most common disorder: acne. Acne is connected to hormones and how they affect the oil glands of the skin.

The skin gets its oil, called sebum, from the sebaceous glands. Each gland connects to a passage of extremely small hairs. The sebum travels through these passages. The oil reaches the surface of the skin through little holes, called pores. Sometimes, the sebum, hair and cells of the pores block these openings. This is how acne starts.

Bacteria can grow in a blocked pore. The bacteria produce chemicals and enzymes. White blood cells -- infection fighters -- travel to the area. All this leads to a growth on the skin, a pimple. This becomes red, hot and often painful.

Some people think eating chocolate or oily foods causes acne. Others blame dirty skin or nervous tension. Yet researchers tell us none of these cause acne.

So what does? Doctors are not sure. But they have some ideas. For one thing, they know that hormones called androgens play a part. Androgens cause the sebaceous glands to grow and make more oil.

Young people will not be happy about this next fact. Androgens increase when boys and girls enter their teenage years.

There are several treatments for acne. Mild cases are generally treated with medicines for use directly on the skin. These often contain salicylic acid or benzoyl peroxide.

People with more serious acne may be given antibiotics to take by mouth. Or they might use a combination of pills and creams.

One of the drugs used to treat the most severe forms of acne is called isotretinoin. It is normally taken for about five months. Isotretinoin has been shown to cure acne in ninety percent of people who use it.

However, isotretinoin and another acne medicine called Accutane can cause serious problems in some cases. If used during pregnancy, for example, they can harm the fetus.

Skin experts say there are simple ways to help prevent acne outbreaks. One is to touch your face as little as possible, so as not to add oils or put pressure on the skin.

Another good idea is to avoid the urge to burst pimples. This can leave permanent marks on the skin. Doctors also say to avoid strong soaps, and to be gentle as you wash and dry your skin.