Milk does not contain pus. What it does contain are somatic cells, which are normal immune cells found in the milk of every healthy mammal. The claim that milk contains pus originated from animal rights campaigns that deliberately equated these immune cells with the medical definition of pus to discourage dairy consumption. The distinction matters, and understanding what’s actually in your glass of milk requires looking at the biology more closely.
What Somatic Cells Actually Are
Somatic cells in milk are primarily white blood cells (leukocytes) that form part of a cow’s immune system. They include three main types: macrophages, which patrol the udder and clean up cellular debris; lymphocytes, which regulate immune responses and identify threats; and neutrophils, which are the first responders when bacteria show up. In a healthy udder, these cells exist at low levels as part of routine immune surveillance. They aren’t a sign of disease, and they aren’t pus.
Pus, by medical definition, is a thick fluid that forms at the site of an active infection. It contains dead white blood cells, dead tissue, and bacteria. The key difference is context: somatic cells circulating through a healthy udder are functioning immune cells doing their normal job. Pus is the aftermath of an infection where tissue has been destroyed. Calling somatic cells “pus” is like calling the white blood cells in your own bloodstream an abscess.
Where the “Pus in Milk” Claim Comes From
The claim gained traction through a campaign by PETA, which used the slogan “Got Pus?” on merchandise and advertisements. The organization pointed to somatic cell counts in milk as evidence that consumers were drinking dangerous levels of pus, linking the cells to an udder infection called mastitis. The dairy industry’s National Milk Producers Federation pushed back, and the dispute played out publicly in the early 2000s.
The framing was strategic. Somatic cells are a real, measurable component of milk, and mastitis is a genuine concern in dairy farming. But the leap from “milk contains immune cells” to “milk contains pus” misrepresents the biology. When a cow does develop mastitis, her somatic cell count rises sharply as neutrophils flood the udder to fight the infection, sometimes making up more than 90% of the cells present. That milk, however, is flagged and handled differently, which brings us to how the system actually works.
How Mastitis Changes the Picture
Mastitis is an inflammation of the udder, usually caused by bacterial infection. When it occurs, the cow’s immune system sends a surge of neutrophils into the mammary gland to fight the invading bacteria. If the immune cells respond quickly and clear the bacteria, the cell count drops back down within days. Macrophages then move in to clean up the remaining bacteria and debris.
This process does temporarily produce milk with a much higher white blood cell count. But dairy operations test milk regularly, and milk from cows with clinical mastitis (visible symptoms like swelling or abnormal milk) is discarded. The somatic cell count of bulk tank milk, which combines milk from the entire herd, is tested before it ever reaches a processing plant.
Regulatory Limits and What Farms Actually Produce
The U.S. legal limit for somatic cells in Grade A milk is 750,000 cells per milliliter, established by the Pasteurized Milk Ordinance. Several states set stricter limits: California caps it at 600,000, Oregon at 500,000, and Idaho and Washington at 400,000.
In practice, most milk falls well below these thresholds. The average somatic cell count for U.S. producer milk in 2023 was around 200,000 cells per milliliter, less than a third of the federal limit. Over the past 24 years, the national average has trended steadily downward, reflecting improvements in herd health and milking practices. A count of 200,000 is generally considered to reflect a herd with good udder health.
To put 200,000 cells per milliliter in perspective, these are microscopic cells suspended in fluid. A milliliter is about one-fifth of a teaspoon. The cells themselves are invisible to the naked eye, and at these concentrations they have no impact on the appearance, texture, or safety of the milk.
What Happens During Processing
Even before pasteurization, industrial milk processing removes a significant portion of somatic cells. Centrifugation, the spinning process used to separate cream, also strips out cells and microorganisms. This step alone reduces somatic cell counts by 35% to 75%. Microfiltration, used for some milk products, removes about 75% of somatic cells from skim milk by trapping them in filter membranes or breaking them apart through the force of processing.
When centrifugation and microfiltration are combined, the reduction is dramatic: 92.6% to 99.5% of somatic cells are removed. So the milk sitting on a grocery store shelf contains a small fraction of the cells that were present when it left the farm. Pasteurization then kills any remaining bacteria, making the final product safe regardless of the original cell count.
Is High-SCC Milk Harmful to Drink?
A comprehensive review by the National Mastitis Council found no documented direct health risk from consuming milk with elevated somatic cell counts. The ingestion of bovine white blood cells has not been shown to harm humans in any scientific study. Your digestive system breaks down these cells the same way it breaks down any other protein.
The one genuine safety concern is unpasteurized milk. When a cow has mastitis, pathogenic bacteria may be present in her milk. Pasteurization is highly effective at killing these organisms, but raw milk skips that step entirely. The risk from high-SCC milk isn’t the cells themselves. It’s the bacteria that might accompany them if the milk isn’t properly heat-treated.
How Somatic Cells Affect Milk Quality
While somatic cells don’t pose a health risk, high counts do affect taste and shelf life. When white blood cells flood the udder during infection, they release enzymes that break down milk components. Some of these enzymes attack milkfat, producing free fatty acids that create a rancid, soapy, or sour flavor. Others break down milk proteins, leading to bitterness.
Cows with mastitis or those late in their lactation cycle can also produce milk with a noticeably salty taste. These flavor defects are one reason the dairy industry monitors somatic cell counts so closely. High-SCC milk doesn’t just signal a sick cow; it produces an inferior product with a shorter shelf life. Farmers are financially penalized for high counts, which creates a strong incentive to keep herds healthy and cell counts low.
The bottom line: milk contains immune cells, not pus. The two are biologically different, the levels in commercial milk are low and tightly regulated, and processing removes the vast majority of what remains. The “pus in milk” framing is an advocacy tool, not a scientific description of what’s in your glass.