Pus

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For the UK punk rock band, see P*U*S. For cats, see puss.

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 white blood 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 neutrophils, 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. After it has killed a bacterium, the neutrophil dies. These dead cells are then phagocytosed by macrophages, which break them down further. Pus, therefore, is the creamy 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.

[edit] Pus in milk

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Most types of milk will contain a small amount of pus.^{[citation needed]} This is completely normal and usually safe unless there is an unusually high amount.

A somatic cell count is carried out before the milk leaves the farm. A dairy farmer's raw milk collection cannot exceed a somatic cell count of 750,000 per ml at the farm gate. Some cows may have udder infections increasing the cell count but these will be diluted as it is added to the other milk. By the time it is bottled up and sold it should contain only around 750,000 per ml, most likely much less.

Pasteurization is used to lower bacterial levels and not to reduce the somatic cell count.