All acne begins with one basic lesion: the comedo, an enlarged hair follicle plugged with oil, dead skin cells and bacteria. Invisible to the naked eye, the comedo lurks beneath the surface of your skin waiting for the right conditions to grow into an inflamed lesion. As the skin continues to produce more oil, bacteria flourish within the swollen follicle. The surrounding skin becomes increasingly inflamed as your white blood cells fight against the intruders.
Though all pimples start the same way, they can take many forms and may react differently for different people. Please note that the following guide is not intended to be used for conclusive self-diagnosis. These definitions may be used to help you decide whether or not you should consider seeking medical attention.
Closed comedo, or whitehead.
If the plugged follicle stays below the surface of the skin, the lesion is called a closed comedo, or whitehead. They usually appear on the skin as small, whitish bumps.
Open comedo, or blackhead.
If the plug enlarges and pushes through the surface of the skin, it's called an open comedo, or blackhead. The plug's dark appearance is not due to dirt, but rather to a buildup of melanin, the skin's dark pigment.
The mildest form of inflammatory acne is the papule, which appears on the skin as a small, firm pink bump. These can be tender to the touch, and are often considered an intermediary step between non-inflammatory and clearly inflammatory lesions.
Like papules, pustules are small round lesions; unlike papules, they are clearly inflamed and contain visible pus. They may appear red at the base, with a yellowish or whitish center. Pustules do not commonly contain a great deal of bacteria; the inflammation is generally caused by chemical irritation from sebum components such as fatty free acids.
Nodule or Cyst.
Large and usually very painful, nodules are inflamed, pus-filled lesions lodged deep within the skin. Nodules develop when the contents of a comedo have spilled into the surrounding skin and the local immune system responds, producing pus. The most severe form of acne lesion, nodules may persist for weeks or months, their contents hardening into a deep cyst. Both nodules and cysts often leave deep scars.
This rare but serious form of inflammatory acne develops primarily on the back, buttocks and chest. In addition to the presence of pustules and nodules, there may be severe bacterial infection.