The direct cause is direct invasion by BACTERIA. Sometimes the presence of foreign bodies, such as bullets or splinters, may produce an abscess if they become contaminated by bacteria.

The micro-organisms most frequently found are staphylococci (see STAPHYLOCOCCUS) and streptococci (see STREPTOCOCCUS). Other abscess-forming organisms are Pseudomonas pyocyanea and Escherichia coli, which may live normally in the bowels and under certain conditions migrate into the surrounding tissues, producing abscesses.

When bacteria have gained access – for example, to a wound – they may multiply rapidly, produce toxins, and cause local dilatation of the blood vessels, slowing of the bloodstream, and EXUDATION of blood corpuscles and fluid. The LEUCOCYTES, (white blood cells) collect around the invaded area and destroy the bacteria either by consuming them (see PHAGOCYTOSIS) or by forming a toxin that kills them. If the body's local defence mechanisms fail to do this, the abscess will spread and may in severe cases cause generalised infection or SEPTICAEMIA.

The classic symptoms of inflammation are redness, warmth, swelling, pain and fever. The neighbouring LYMPH NODES may become swollen and tender in an attempt to stop the bacteria spreading to other parts of the body. Infection also causes an increase in the number of leucocytes in the blood (see LEUCOCYTOSIS). Immediately the abscess is opened or bursts, the pain disappears, the temperature falls rapidly to normal, and healing proceeds. If, however, the abscess discharges into an internal cavity such as the bowel or bladder, it may heal slowly or become CHRONIC, resulting in increasing ill-health.

Abscesses can occur in any tissue in the body, but the principles of treatment are broadly the same: use of ANTIBIOTICS and, where appropriate, surgery to drain the pus.