Pus is a thick fluid made of dead immune cells, destroyed tissue, bacteria (dead or alive), and tissue fluids. It forms when your immune system sends waves of white blood cells to fight an infection, and those cells die in the process. What you see as pus is essentially the aftermath of that battle.
The Main Ingredients
The single largest component of pus is dead or dying neutrophils, a type of white blood cell that serves as your body’s first responder to infection. When bacteria invade tissue, your immune system floods the area with neutrophils. These cells engulf and kill bacteria, but most of them die in the process. Their accumulated remains form the bulk of what you recognize as pus.
Mixed in with those dead neutrophils is tissue debris. The infection itself, combined with enzymes released by dying cells, breaks down surrounding tissue into a soft, paste-like material. This breakdown happens through a process where powerful enzymes from both dead cells and immune cells dissolve solid tissue into liquid. The result is a slurry of dissolved cellular material that gives pus its characteristic thick, fluid consistency.
The remaining ingredients are bacteria (both live and dead), tissue fluids that seep out of nearby blood vessels during inflammation, and various proteins from blood serum. Together, these components create the opaque, viscous substance that collects at the site of infection.
Why Pus Has Color
Pus ranges from whitish-yellow to green, and the color comes from specific sources. The yellow-green tint is largely caused by an enzyme called myeloperoxidase, which is packed inside neutrophils. This enzyme contains a pigment similar to the one in blood, but instead of being red, it’s green. It was originally named “verdeperoxidase” (verde meaning green) before scientists renamed it after discovering where it’s produced. When millions of neutrophils die and release their contents, that green pigment accumulates and becomes visible.
Certain bacteria can shift the color further. One well-known example is Pseudomonas aeruginosa, a bacterium that produces a blue-green pigment called pyocyanin. Infections caused by this organism often produce distinctly green or blue-green pus, which can be a useful clue for identifying the type of infection.
Which Bacteria Cause Pus to Form
Not all bacteria trigger pus formation. The ones that do are called pyogenic bacteria, from the Greek word for “pus-producing.” The most common culprits are Staphylococcus and Streptococcus species, which are responsible for the majority of skin infections, boils, and abscesses people encounter. Other frequent pus-producers include Neisseria and Haemophilus species.
These bacteria cause damage through a toolkit of harmful substances: enzymes that break down proteins, toxins that rupture red blood cells, and molecules that help the bacteria evade your immune system. The more aggressive the bacterial invasion, the more neutrophils your body sends, and the more pus accumulates as those neutrophils are destroyed.
Where Pus Collects in the Body
When pus builds up in a walled-off pocket within tissue, it forms an abscess. Your body essentially walls off the infection with a layer of inflammatory tissue, trapping the pus inside. Abscesses can develop almost anywhere: under the skin, in the gums, around the tonsils, or inside organs like the liver or lungs. They tend to stay in one place and often need to be drained.
Pus can also accumulate in body cavities that already exist. When it fills the space between the lung and chest wall, the condition is called empyema. The distinction matters because an abscess is a new pocket the body creates, while empyema is pus filling a pre-existing space. On a smaller scale, a boil (furuncle) is simply an abscess centered around a hair follicle.
Pus Without Infection
Not all pus means bacteria are involved. Sterile pus, which contains the same dead immune cells and tissue debris but no bacteria, forms in several conditions. The autoimmune skin disease pemphigus foliaceus produces pus-filled blisters driven entirely by a misdirected immune response. Certain types of acne, juvenile cellulitis, and eosinophilic pustulosis also produce sterile pus. Physical trauma, surgery, and chemical injury can all trigger neutrophils to flood an area and die in large numbers, creating pus even though no infection is present.
This is an important distinction because sterile pus won’t respond to antibiotics. Its presence signals inflammation rather than infection, and the underlying cause needs different treatment.
How Your Body Clears Pus
Small amounts of pus from minor infections are handled by the body on its own. Macrophages, a second wave of immune cells, arrive at the scene and begin consuming the debris. They digest dead neutrophils, bacterial remains, and dissolved tissue, gradually clearing the area so healing can begin. For a small infected cut or pimple, this cleanup process resolves without intervention over days to a week or two.
Larger collections are a different story. An abscess with a significant pocket of pus often cannot be cleared by the body’s cleanup crew alone, because the walled-off capsule prevents adequate blood flow and immune cell access to the center. This is why abscesses frequently need to be drained, either by opening naturally through the skin or with medical help. Until the pus is removed and the cavity can collapse, healing stalls.