How Does Acne Develop?

Doctors describe acne as a disease of the pilosebaceous units. Found over
most of the body, pilosebaceous units consist of a sebaceous (oil) gland
connected to a hair-containing canal called a follicle (see below). These units
are largest and most numerous on the face, upper back, and chest—areas
where acne tends to occur. The sebaceous glands make an oily substance
called sebum that normally empties onto the skin surface through the
opening of the follicle.
Acne is believed to result from a change in the inner lining of the follicle that
prevents the sebum from passing through. For reasons not understood, cells
from the lining of the follicle are shed too fast and clump together. The
clumped cells plug up the follicle’s opening so sebum cannot reach the
surface of the skin. The mixture of oil and cells causes bacteria that normally
live on the skin, called Propionibacterium acnes (P. acnes), to grow in the
plugged follicles. These bacteria produce chemicals and enzymes that can
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 plugged follicle can no longer hold its contents, it bursts and
spills everything onto the nearby skin—sebum, shed skin cells, and bacteria.
Lesions or pimples develop as a result of the skin’s being irritated.

People with acne frequently have a variety of lesions, some of which are
shown below. The basic acne lesion, called the comedo (kom´-e-do) or
comedone, is simply an enlarged hair follicle plugged with oil and bacteria.
This lesion is often referred to as a microcomedo because it cannot be seen by
the naked eye. If the plugged follicle, or comedo, stays beneath the skin, it is
called a closed comedo or whitehead. Whiteheads usually appear on the skin
surface as small, whitish bumps. A comedo that reaches the surface of the
skin and opens up is called a blackhead because it looks black on the skin’s surface. This black discoloration 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)—inflamed, pus-filled lesions that can be red at the
base.
       Nodules—large, painful, solid lesions that are lodged deep within the
skin.
       Cysts—deep, inflamed, pus-filled lesions that can cause pain and
scarring.