An abscess is a collection of pus that has accumulated in a cavity formed by the tissue on the basis of an infectious process (usually caused by bacteria or parasites) or other foreign materials (e.g. splinters or bullet wounds). It is a defensive reaction of the tissue to prevent the spread of infectious materials to other parts of the body.
The organisms or foreign materials that have gained access to a part of tissue kill the local cells, resulting in the release of toxins. The toxins trigger an inflammatory response, which draws huge amounts of white blood cells to the area and increases the regional blood flow.
The final structure of the abscess is an abscess wall that is formed by the adjacent healthy cells in an attempt to build a barrier around the pus that limits the infected material from neighboring structures and also limits immune cells from attacking the bacteria.