pus

Pus is a viscous, opaque fluid produced during certain inflammatory processes, especially bacterial infections. It consists mainly of dead and dying neutrophils, tissue debris, microbes, and fluid rich in proteins. Its formation is part of the body's acute inflammatory response, driven by the recruitment of neutrophils to the site of infection and their subsequent phagocytosis and destruction of invading pathogens.

Pus can accumulate in localized collections called abscesses, where the pus becomes walled off by fibrous tissue. It may also be discharged from wounds or formed within infected cavities such as sinuses. The appearance of pus varies; it is typically pale yellow but can be green or brown depending on the age of the pus and the types of bacteria present. Green coloration is sometimes attributed to the enzymes and pigments released by neutrophils, while foul odors may occur with anaerobic infections.

Pus is a clinical sign of a purulent or pyogenic infection. Not all infections produce pus, and the absence of visible pus does not rule out infection. The body's response aims to contain infection and limit tissue damage, but when pus collects in a closed space an abscess may form and require drainage for resolution, alongside antimicrobial therapy when appropriate.