Pus is anexudate, typically white-yellow, yellow, or yellow-brown, formed at the site ofinflammation during infections, regardless of cause.[1][2] An accumulation of pus in an enclosed tissue space is known as anabscess, whereas a visible collection of pus within or beneath theepidermis is known as apustule,pimple or spot.
Pus consists of a thin,protein-rich fluid (historically known asliquor puris[3][4]) and deadleukocytes (white blood cells) from the body'simmune response (mostlyneutrophils).[5] During infection,T helper cells releasecytokines, which trigger neutrophils to seek the site of infection bychemotaxis. There, the neutrophils releasegranules, which destroy the bacteria. The bacteria resist the immune response by releasingtoxins calledleukocidins.[6] As the neutrophils die off from toxins and old age, they aredestroyed by macrophages, forming the viscous pus. Bacteria that cause pus are called pyogenic.[6][7]
Pus with blood coming out of ring finger
Although pus is normally of a whitish-yellow hue, changes in the color can be observed under certain circumstances. Pus is sometimes green because of the presence ofmyeloperoxidase, an intensely green antibacterial protein produced by some types of white blood cells. Green, foul-smelling pus is found in certain infections ofPseudomonas aeruginosa. The greenish color is a result of the bacterial pigmentpyocyanin that it produces.Amoebic abscesses of theliver produce brownish pus, which is described as looking like "anchovy paste". Pus from anaerobic infections can more often have a foul odor.[8]
In almost all cases when there is a collection of pus in the body, a clinician will try to create an opening to drain it. This principle has been distilled into the famousLatinaphorism "Ubi pus, ibi evacua" ("Where there is pus, evacuate it").
In the pre-asepsis era, surgeonFrederick Treves (1853–1923) wrote,"Practically all major woundssuppurated. Pus was the most common subject of converse [among surgeons], because it was the most prominent feature in the surgeon's work. It was classified according to degrees of vileness." [11]: 347 But pus of the right kind was considered desirable.[12]: 80 "If a patient was lucky... a thick cream-colored odorless fluid would appear within five or six days";such "laudable" pus was considered "a sure sign that the wound would heal" [11]: 344 because it meant "Nature has put up a bold fight against the invader".[13]"On the other hand, if the pus gradually became watery, blood tinged and foul smelling, it was designated 'sanious' [14][or 'ill-conditioned' ] [15]and the wound condition was considered unfavorable".[14]It later came to be understood that "laudable" pus generally implied an invasion of relatively benignstaphylococcus, while "ill-conditioned" pus usually meant the more dangerousstreptococcus was present.[11]: 345 [14]: 247
^Van Hoosen, Bertha (Autumn 1947)."A Woman's Medical Training in the Eighties".Quarterly Review of the Michigan Alumnus: A Journal of University Perspectives. University of Michigan Libraries:77–81. UOM:39015006945235.
