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Pus is a whitish-yellow or yellow substance produced during inflammatory responses of the body that can be found in regions of pyogenic bacterial infections. An accumulation of pus in an enclosed tissue space is known as an abscess. A visible collection of pus within or beneath the epidermis, on the other hand, is known as a pustule or pimple. Pus is produced from the dead and living cells which travel into the intercellular spaces around the affected cells.

Something that creates pus is called suppurative, pyogenic, or purulent. If it creates mucus as well as pus, it is called mucopurulent.

Pus consists of a thin, protein-rich fluid, known as liquor puris, and dead cells, which are part of the body's innate immune response. Neutrophils are produced in the bone marrow and released into the blood. When the need to fight infection arises, they move to the site of infection by a process known as chemotaxis, usually triggered by cytokine release from macrophages that sense invading organisms. At the site of infection they engulf and kill bacteria. Eventually, the neutrophils die, and these dead cells are then phagocytosed by macrophages, which break them down further. Pus, therefore, is the viscous material composed of these dead neutrophils.

Neutrophils are the most abundant type of leukocyte in human blood, comprising anywhere between 40% to 75% of leukocytes.

When seen in a wound or dry skin, pus indicates the area is infected and should be cleaned with antiseptic.

Despite normally being of a whitish-yellow hue, changes in the color of pus can be observed under certain circumstances. Blue pus is found in certain infections of Pseudomonas aeruginosa as a result of the pyocyanin bacterial pigment it produces; amoebic abscesses of the liver, meanwhile, produce brownish pus. Pus might have a reddish tint to it after mixing with blood. Pus also can have an odor which is pleasing to some but irritating to others.

• Ubi pus, ibi evacua