Pus is a viscous substance that is a part of the body’s natural immune response system. It is most often whitish-yellow in color, although it may also be greenish, brownish, reddish, or even blue. Pus often has a somewhat necrotic smell, and is often the sign of an infection when found in a wound.
When the body detects some sort of foreign infection, it immediately begins a response to neutralize the invader and limit damage to the system. White blood cells, or leukocytes, are the cells responsible for immune response in the body, and are produced in the marrow of bones. The majority of these white blood cells are a type known as neutrophil, which are tasked with attacking foreign bacteria and fungi.
When a foreign invader is detected, leukocytes known as macrophages detect the invasion and release cytokines that act as an alarm system. This alerts neutrophils of the invader, and they begin to make their way towards the site of the infection. They begin their journey through blood vessels, and then through the interstitial tissue itself. A full neutrophilic response is usually underway in less than an hour after a wound appears, and is the cause of the basic immune response that ultimately results in pus.
When the neutrophils get to the source of the infection, they begin to eat the offending particulate matter of microorganisms, in a process known as phagocytosis. When they ingest these microbes, they kill them, helping to clean up an infection site. The lifespan of a neutrophil is about twelve hours, and so eventually they die off, while still encompassing the now inert matter they were protecting against. Macrophages then break down the dead neutrophils, which are combined with liquor puris to create the substance. It is then expelled from the body, taking the dead cells and inert matter with it.
The natural color in a standard immune response is a pale yellow. The substance is most normally seen in the epidermis itself, or else just below the epidermal layer, in which case it is referred to as either a pimple or a pustule. Pus may also build up within a fairly rigid space, in which case it forms an abscess. It may take on colors other than pale yellow, depending on the response that has been triggered.
If the substance has at some point mixed with blood, it will often have a reddish tinge, but this is a superficial coloring, and not related to the actual process of fighting off infection. Brownish pus will sometimes form when the liver is abscessing, usually as the result of an amoebic attack. Green pus may also form, which simply reflects a high level of myeloperoxidase in the neutrophils; this is an antibacterial protein naturally secreted by neutrophils, which has a vivid green color. In rare cases, the substance may be blue, usually when the neutrophils have been active in fighting off the bacterium Pseudomonas aeruginosa.
In nearly all cases, pus is not something to be worried of in itself, although it is indicative of an underlying infection. It's a sign of an active immune system doing its role, and should simply be cleared away regularly. Topical solutions can be used to help assist in the fighting off of a bacterial infection, and if it continues forming in the same location over long periods of time, it is probably reflective of a chronic infection that should be looked at by a physician.