Why do abscesses smell so bad?

Abscesses may be caused by many different bacteria, fungi, protozoans or by other foreign materials (e.g. a grass seed).

An abscess is a collection of pus (dead neutrophils) that forms a cavity in tissue. It is a defensive reaction designed to prevent the spread of infectious materials. The presence of these organisms damage and kill surrounding cells. The cells release cytokines - chemicals that trigger the inflammatory response. They attract neutrophils and other white blood cells to the area. They also increase the local blood flow resulting in a red appearance (erythema) and feeling of heat. As pus collects, a wall or capsule is formed by the nearby healthy cells in order to prevent the infection spreading to surrounding tissue. This capsule also tends to keep other immune cells from getting at the causative organisms.

This multiplication of organisms and the cellular destruction also forms a cocktail of some unpleasant chemicals. The exact mix depends on the organisms. Gasses such as ammonia, methane and sulphur dioxide (think rotten eggs) may be produced. Two other compounds are largely responsible for the foul odour of putrefying flesh. The delightfully named putrescine and cadaverine, are produced by the breakdown of amino acids in the tissue.

But what is the point of all this stink? To us the smell is disgusting and we are repelled by it. We instinctively avoid anything that creates such odours helping to protect us from exposure to infectious agents.