A pimple is a small pocket of oil, dead skin cells, bacteria, and (if it’s inflamed) immune cells trapped inside a clogged hair follicle. The exact mix depends on the type of blemish, from a simple blackhead to a deep, painful cyst. Here’s what’s actually going on beneath the surface.
The Plug: Oil and Dead Skin
Every pimple starts with a clog. Your skin constantly produces sebum, a light yellow, waxy fluid that keeps your skin moisturized. Sebum is a mix of triglycerides, free fatty acids, squalene, wax esters, and cholesterol. On its own, it’s not a problem. The trouble begins when dead skin cells inside a hair follicle don’t shed the way they’re supposed to. Instead of sloughing off and clearing out, they clump together with sebum, forming a sticky plug called a microcomedone.
This plug blocks the narrow opening of the follicle. Sebum keeps being produced behind the blockage, and with nowhere to go, it builds up and stretches the follicle like a tiny balloon. At this stage, the contents are mostly oily sebum mixed with clumps of a tough protein called keratin, the same material your hair and nails are made of.
Blackheads vs. Whiteheads
If the clog stays sealed beneath the surface of your skin, it’s a whitehead (closed comedone). The trapped material remains a pale, soft mass of oil and dead cells. If the follicle opening widens and the plug reaches the air, you get a blackhead (open comedone). The dark color isn’t dirt. It’s the result of oxidation: oxygen reacts with the exposed material at the surface and turns it dark, much like a sliced apple browns when left out.
Neither blackheads nor whiteheads contain pus. They’re non-inflammatory, meaning your immune system hasn’t gotten involved yet. The contents are purely structural: sebum and compacted dead skin.
Where Bacteria Fit In
A bacterium called Cutibacterium acnes (formerly known as Propionibacterium acnes) lives naturally on almost everyone’s skin. It feeds on the fats in sebum. In a healthy, open pore, this is harmless. But inside a sealed, oil-rich comedone, conditions are ideal for these bacteria to multiply rapidly.
Not all strains of this bacterium cause problems equally. Research using whole-genome sequencing has found that certain strains carry a specific genetic element, a linear plasmid, that triggers a much stronger inflammatory response in surrounding tissue. These high-inflammatory strains are found more often in acne lesions, while milder strains tend to dominate on healthy skin. So the bacterium itself isn’t the enemy. The specific strain, combined with a clogged pore, is what tips a quiet comedone toward a red, painful pimple.
What Pus Actually Is
When bacteria multiply inside a blocked follicle, your immune system responds by flooding the area with white blood cells, primarily neutrophils and macrophages. These cells attack the bacteria and any tissue they identify as damaged. In the process, both the immune cells and the bacteria die in large numbers. The dead cells break down, leaving behind small pockets of fluid.
That fluid is pus. It’s a mixture of dead and dying neutrophils, dead macrophages, destroyed bacteria, broken-down tissue, and leftover sebum. The whitish or yellowish color comes from the accumulated immune cells. A pimple with a visible white or yellow head (a pustule) is essentially a tiny battlefield where your immune system has already engaged and casualties are pooling near the surface.
Deeper inflammation without a clear head, the kind that feels like a hard, tender bump, is called a papule. It contains the same immune response but hasn’t formed a defined pocket of pus yet. The redness and swelling you see is caused by your body dilating blood vessels in the area to rush in more immune cells.
What’s Inside Cystic Acne
Cystic acne forms deep under the skin rather than near the surface. Unlike a standard pimple, a cyst develops within a membrane or sac-like structure. Inside, you’ll find a larger collection of the same ingredients: sebum, dead cells, bacteria, and inflammatory fluid. But because the lesion sits so far below the surface, the pressure and inflammation affect deeper layers of tissue.
This depth is what makes cystic acne so painful. The swelling puts pressure on surrounding nerves in a way that shallow pimples don’t. It’s also why cystic lesions are far more likely to leave lasting scars. The inflammation damages the structural layer of skin (the dermis), and the repair process often produces uneven collagen, resulting in pitted or raised scars.
Why Popping Makes Things Worse
When you squeeze a pimple, some of the contents may come out, but some almost always gets pushed deeper into the surrounding skin. According to the American Academy of Dermatology, this drives bacteria, dead cells, and inflammatory material further into the dermis, intensifying the immune response. The result is more redness, more swelling, and a higher chance of scarring or dark spots that can linger for months.
The contents of a pimple are, by definition, material your body is actively trying to break down and clear. Given time, your immune system will reabsorb the debris, rebuild the tissue, and resolve the blemish on its own. Forcing that process by squeezing essentially reopens the battlefield and spreads the damage to previously uninvolved tissue.
How Common This Is
Acne affects roughly 85% of people between ages 12 and 25, and about 80% of people will deal with it at some point between ages 11 and 30. It’s not just a teenage problem: up to 20% of adult women and 8% of adult men continue to experience breakouts well past adolescence. In the United States alone, an estimated 50 million people are affected. Globally, acne’s overall prevalence sits around 9.4%, making it one of the most common skin conditions on earth.