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Pus is an exudate, typically white-yellow, yellow, or yellow-brown, formed at the site of inflammation during infection. An accumulation of pus in an enclosed tissue space is known as an abscess, whereas a visible collection of pus within or beneath the epidermis is known as a pustule or pimple.
Pus consists of a thin, protein-rich fluid, known as liquor puris, and dead leukocytes from the body's immune response (mostly neutrophils). During infection, macrophages release cytokines which trigger neutrophils to seek the site of infection by chemotaxis. There, the neutrophils engulf and destroy the bacteria and the bacteria resist the immune response by releasing toxins called leukocidins. As the neutrophils die off from toxins and old age, they are destroyed by macrophages, forming the viscous pus.
Despite normally being of a whitish-yellow hue, changes in the color of pus can be observed under certain circumstances. Pus is sometimes green because of the presence of myeloperoxidase, an intensely green antibacterial protein produced by some types of white blood cells. Green, foul-smelling pus is found in certain infections of Pseudomonas aeruginosa. The greenish color is a result of the bacterial pigment pyocyanin that it produces. Amoebic abscesses of the liver produce brownish pus, which is described as looking like "anchovy paste". Pus can also have a foul odor, particularly pus from anaerobic infections.
In almost all cases when there is a collection of pus in the body, the clinician will try to create an opening for it to evacuate, This principle has been distilled into the famous Latin aphorism "Ubi pus, ibi evacua" ("Where there is pus, there evacuate it").
Staphylococcus aureus bacteria is the most common cause of boils.