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 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 stored in bone marrow and are released when the need to fight infection arises, usually triggered by cytokine release from macrophages that sense invading organisms. They then travel to the infected tissue, engulfing and killing 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, for anywhere from 40% to 75% of leukocytes.
When seen in a wound or dry skin, pus indicates it is infected, and should be cleaned with antiseptic.
Something that creates pus is called suppurative, pyogenic, or purulent. If it creates mucus as well as pus, it is called "Mucopurulent".
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; amebic abscesses of the liver, meanwhile, produce brownish pus.