Acne: Localized skin inflammation as a result of overactivity of the oil glands at the base of hair follicles. 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

Signs and symptoms

Acne typically appears on your face, neck, chest, back and shoulders, which are the areas of your skin with the largest number of functional oil glands. Acne can take the following forms:

Comedones (whiteheads and blackheads).
Comedones (kom-uh-DO-neze) are created when the openings of hair follicles become clogged and blocked with oil secretions, dead skin cells and sometimes bacteria. When comedones are open at the skin surface they're called blackheads because of the dark appearance of the plugs in the hair follicles. When comedones are closed, they're called whiteheads — slightly raised, skin-colored bumps.

• Papules. These are small raised bumps that signal inflammation or infection in the hair follicles. Papules may be red and tender.
• Pustules. Similar to papules, pustules are red, tender bumps with white pus at their tips.
• Nodules. These are large, solid, painful lumps beneath the surface of the skin. They're formed by the buildup of secretions deep within hair follicles.
• Cysts. These are painful, pus-filled lumps beneath the surface of the skin. These boil-like infections can cause scars.