What Causes Pus?

Brendan McGuigan

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.

Pus is typically a sign of infection in a wound.
Pus is typically a sign of infection 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.

Pseudomonas aeruginosa bacteria, which can cause blue pus.
Pseudomonas aeruginosa bacteria, which can cause blue pus.

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.

A diagram showing different types of white blood cells. Pus is mostly made up of dead white blood cells.
A diagram showing different types of white blood cells. Pus is mostly made up of dead white blood cells.

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.

Pus located just below the epidermal layer can be referred to as a pimple.
Pus located just below the epidermal layer can be referred to as a pimple.

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.

There are a number of cleansing products on the market to help reduce the occurrence of pimples.
There are a number of cleansing products on the market to help reduce the occurrence of pimples.

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Discussion Comments


As a lab technician, I have seen lots of pus! It pays well.


I hate it when I squeeze a pimple and watery blood just keeps oozing out after the white pus has evacuated the pore. Sometimes, but rarely, the pus will be all that comes out, and then the pore will be dry. However, most of the time, I'm dealing with the watery after-effects of squeezing.

This makes it hard to put on makeup. It just drips off my face with the liquid. I have to put a tiny piece of tissue on the area to help it clot, like my dad does when he cuts himself shaving!


My dog recently had an unpleasant experience that led to the development of lots of pus. He got attacked by the neighbor's dog, and what started out as a small bite wound led to swelling in the lymph nodes and a seemingly endless supply of dripping pus.

For the first few days, he seemed fine. After that, he started getting swollen spots on his underside, and when I saw that the wound appeared to be liquefying and oozing off his body, making a puddle of pink and brown pus, I took him to the vet.

She had to lance the wound to get the pus out. After that, I had to clean it with bleach and water every day, and I had to keep him inside so that flies didn't try to lay eggs in his wound, which needed to heal from the inside out, so it had no scab to protect it. Also, he was on antibiotics for fourteen days.


@shell4life – It's probably a sign of a urinary tract infection. I've had pus cells in my urinalysis before, and this was always my diagnosis.

I also had cramping in my bladder, and I felt the need to urinate every half hour. It burned a little when I did, too.

Since pus indicates infection, there were bacteria involved. My doctor gave me antibiotics to make it all better.

I've learned that drinking cranberry juice every day and replacing most of my other drinks with water helps prevent me from getting urinary tract infections. It can also prevent kidney stones from forming, and they are another thing that might cause pus to show up on a urine test.


I was waiting in an exam room at my doctor's office the other day, and I could hear my doctor talking to a patient in the next room. I'm almost sure I heard her tell the person that she found pus cells in her urine.

I had no idea this was even possible! What could it indicate?


I had a boil, and I busted it a while ago. Now it's small. Then I tried to see if it had more pus and it did. Then it got bigger. I busted it and it got bigger until it went down. I tried to prick it again, now it's bigger. What to do?


Just a little FYI: When you have a pimple on your face, it contains pus. When you squeeze a pimple, you should always wash your hands afterwards because the pus could still contain some live bacteria and can be spread to other parts of the body.


While pus isn’t very pleasant to look at or smell, it is actually a good thing. It’s a sign that your body is fighting off infection.

When bacteria attack the body, white blood cells are drawn to the site of the infection. During this “fight”, the bacteria are killed but the body is left with debris. That’s what pus is. Pus contains dead neutrophils, what’s left of pathogens, and even dead skin cells.


@momm44: My suggestion would be to spray some Shout on them and pretreat them. Then, wash the clothes in hot water. You should use some color-safe bleach and pour that in with the clothes. Do not wash anything else with them.


My boyfriend has a boil that ruptured. how do I clean pus from his clothes?

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