A collection of pus, dead neutrophilsi, that has concentrated in a cavity formed by the tissue on the basis of an infectious process or other foreign materials. They are caused by an infection developing in the body's tissue and the body is unable to isolate the infection to keep it from spreading. This is a defensive reaction of the tissue to prevent the spread of infectious materials to other parts of the body. The invasion of the body results in the killing of local cells which in turn release toxins triggering inflammation drawing large numbers of white blood cells. A wall, or capsule is formed by the adjacent healthy cells in an attempt to keep the pus from infecting the rest of the body. The encapsulation prevents immune cells from attacking bacteria in the pus, or from reaching the causative organism or foreign object. If left untreated they can cause serious medical complications including death.