[Extracted from the web page of www.wisegeek.com]
A pustule is a vesicle filled with pus and located under the surface of the skin. Pustules are probably most commonly associated with acne, a common dermatological condition for teenagers, although they are linked with other medical issues. While a pustule may look unpleasant, it is often benign, as long as it is well cared for. Recurrent pustules, pustules which grow large, or pustules which develop signs of additional infection and inflammation may require the attention of a dermatologist.
Pustules can form inside the layers of the epidermis, or just below it, in the dermis. They are filled with a collection of dead cells which are in the process of breaking down. If a pustule is ruptured, a thick white fluid will ooze out. White blood cells are typically abundant in a pustule, in contrast with normal vesicles, which are simply filled with fluid, and no necrotic inflammatory cells.
Many pustules will resolve themselves on their own. The dead cells may be reabsorbed and expressed, or the pustule will eventually rupture, allowing the pus to drain. The site of the pustule will eventually heal, with the skin returning to normal. However, if a pustule becomes severely inflamed, it can cause a scar, and scars can also occur when pustules keep occurring in the same spot, or when people pick at a pustule.
Patients should be aware that not all pustules are acne related. Some are associated with certain types of rashes, and a sudden outbreak of pustules can be a sign that someone has an underlying disease or condition which merits a trip to the doctor. A doctor can examine the pustule to determine the cause and make recommendations about treatment. Pustules can also form when there is a foreign body under the skin which the body cannot get rid of, such as a splinter which leads to inflammation and a subsequent buildup of pus.