A periapical abscess is a collection of pus at the root of a tooth, usually caused by an infection that has spread from a tooth to the surrounding tissues.
The body attacks an infection with large numbers of white blood cells. Pus is the accumulation of these white blood cells, dead tissue, and bacteria. Usually, pus from a tooth infection spreads from the root tip through the bone into the gums so the gums swell near the root of the tooth. The swelling from the pus is often the cause of intense constant pain that worsens when chewing. Depending on the location of the tooth, the infection may spread further into soft tissues (cellulitis), causing swelling in the jaw, into the floor of the mouth, or in the area of the cheeks. Eventually, the tissue may break open, allowing the pus to drain.
Dentists treat an abscess by draining the pus, which requires oral surgery or root canal treatment. Antibiotics help eliminate the infection, but removing the diseased pulp and draining the pus are more important.