An abscess is a collection of pus (dead neutrophils) that has accumulated in a cavity formed by the tissue in which the pus resides due to an infectious process (usually caused by bacteria or parasites) or other foreign materials (e.g., splinters, bullet wounds, or injecting needles). It is a defensive reaction of the tissue to prevent the spread of infectious materials to other parts of the body. One example of an abscess is a BCG-oma, which is caused because of incorrect administration of the BCG vaccine.