According to the American Academy of Dermatology, the four basic mechanisms contributing to acne are hormones, increased sebum production, changes inside hair follicles, and bacteria. Acne usually occurs at age 11 to 14 when the body starts producing male hormones called androgens. Androgens can over-stimulate sebaceous glands and make them produce more sebum.

Dead cells inside hair follicles normally are shed and come out onto the surface of the skin. But in people with acne, the cells are shed faster, stick together, mix with sebum, and clog the follicle. Then bacteria contaminate the skin cell and sebum mixture and grow. When the body’s immune system tries to destroy the bacteria, inflammation results.

Source: U.S. Food and Drug Administration

Another factor is heredity or genetics. Researchers believe that the tendency to develop acne can be inherited from parents. For example, studies have shown that many school-age boys with acne have a family history of the disorder. Certain drugs, including androgens and lithium, are known to cause acne. Greasy cosmetics may alter the cells of the follicles and make them stick together, producing a plug.

The hair, sebum, and keratinocytes that fill the narrow follicle may produce a plug, which is an early sign of acne. The plug prevents sebum from reaching the surface of the skin through a pore. The mixture of oil and cells allows bacteria Propionibacterium acnes (P. acnes) that normally live on the skin to grow in the plugged follicles. These bacteria produce chemicals and enzymes and attract white blood cells that cause inflammation. (Inflammation is a characteristic reaction of tissues to disease or injury and is marked by four signs: swelling, redness, heat, and pain.) When the wall of the plugged follicle breaks down, it spills everything into the nearby skin – sebum, shed skin cells, and bacteria – leading to lesions or pimples.

People with acne frequently have a variety of lesions, some of which are shown in the diagrams below. The basic acne lesion, called the comedo (KOM-e-do), is simply an enlarged and plugged hair follicle. If the plugged follicle, or comedo, stays beneath the skin, it is called a closed comedo and produces a white bump called a whitehead. A comedo that reaches the surface of the skin and opens up is called an open comedo or blackhead because it looks black on the skin’s surface. This black discoloration is due to changes in sebum as it is exposed to air. It is not due to dirt. Both whiteheads and blackheads may stay in the skin for a long time.

Other troublesome acne lesions can develop, including the following:

• Papules – inflamed lesions that usually appear as small, pink bumps on the skin and can be tender to the touch
• Pustules (pimples) – papules topped by white or yellow pus-filled lesions that may be red at the base
• Nodules – large, painful, solid lesions that are lodged deep within the skin
• Cysts – deep, painful, pus-filled lesions that can cause scarring.