A zit is a pocket of oil, dead skin cells, bacteria, and (if it’s inflamed) immune cells trapped inside a clogged pore. The exact mix depends on the type of blemish. A blackhead is mostly hardened oil and skin debris. A red, swollen pimple adds bacteria and white blood cells to that mix. A deep cyst contains all of the above, suspended in fluid. Here’s what’s actually going on inside each layer.
The Oil That Starts It All
Your skin constantly produces an oily substance called sebum through tiny glands attached to hair follicles. Sebum is a complex blend of fats: triglycerides, wax esters, a compound called squalene, and free fatty acids. In people with acne, the composition shifts in ways that make breakouts more likely. Compared to clear skin, acne-prone skin produces sebum with about 34% more squalene, 19% more triglycerides, and significantly fewer free fatty acids. That altered oil is thicker and more likely to clog pores rather than flowing smoothly to the skin’s surface.
Dead Skin Cells That Won’t Let Go
The lining of every pore is made of skin cells called keratinocytes. Normally, these cells shed in an orderly way and get pushed out with the oil. In acne-prone skin, that process breaks down. The cells stick together instead of sloughing off, forming a dense plug of protein and debris at the top of the pore. This plug is the foundation of every zit, from a tiny bump you can barely see to a full-blown cyst. Think of it like a drain cover made of compacted dead skin, sealed in place by sticky oil.
This initial blockage, sometimes called a microcomedone, is invisible to the naked eye. It can exist under the surface for weeks before it either resolves on its own or grows into a visible blemish.
Blackheads vs. Whiteheads
Once a pore is plugged, it becomes one of two things depending on whether the opening stays wide or narrows shut. If the plug stays beneath a thin layer of skin, you get a whitehead: a small, closed bump filled with trapped sebum and dead cells. No bacteria have triggered an immune response yet, so there’s no redness or pain.
If the pore opening remains wide, the top of the plug sits exposed to air. Oxygen reacts with the lipids in the sebum and with melanin (the same pigment that colors your skin), turning the surface dark brown or black. That’s a blackhead. The color has nothing to do with dirt. You could scrub your face raw and the blackhead would still be dark, because the discoloration is a chemical reaction happening within the plug itself.
Where Bacteria Fit In
A bacterium called Cutibacterium acnes (formerly known as Propionibacterium acnes, renamed in 2018) lives naturally on everyone’s skin. It feeds on sebum. When a pore gets blocked, conditions inside become low-oxygen and oil-rich, which is exactly what this bacterium thrives in. It produces an enzyme that breaks down triglycerides in sebum into glycerol, which it uses as fuel, and free fatty acids, which irritate the pore lining.
Acne-associated strains of this bacterium are especially active. They produce higher amounts of propionate, a short-chain fatty acid that lowers the pH inside the pore and provokes a strong immune response. They also generate compounds called porphyrins that further inflame surrounding tissue. This is the transition point where a simple clogged pore becomes a red, angry pimple.
What Pus Actually Is
When bacteria multiply inside a blocked pore, your immune system sends white blood cells to fight the infection. Two types do most of the work: neutrophils, which arrive first and attack bacteria aggressively, and macrophages, which clean up the aftermath. These immune cells kill bacteria but also destroy some of the surrounding tissue in the process. The battlefield of dead immune cells, dead bacteria, dissolved tissue, and leftover fluid is what we call pus.
Pus is typically whitish-yellow. That color comes from the neutrophils and macrophages themselves. In the early hours of a new pimple, you won’t see much of it. Research on early acne lesions shows that in the first 6 to 72 hours, a new blemish is just a small bump with mild redness and no visible pus. After about 72 hours, neutrophils appear in roughly a third of lesions, and the characteristic white head begins to form.
Why Some Zits Hurt
The pain you feel from an inflamed pimple comes from inflammatory chemicals released during the immune battle. Cytokines, chemokines, and certain lipid molecules flood the area around the clogged pore. These chemicals bind to pain-sensing nerve endings in your skin, making them fire more easily. That’s why even lightly touching an inflamed pimple can feel disproportionately painful: the nerve endings in that spot have been chemically sensitized to register pressure as pain.
Deeper pimples hurt more because they press against a denser network of nerves in the lower layers of skin, and the inflammation has more room to spread before it reaches the surface.
What’s Inside a Cyst
Cystic acne forms when the wall of a deeply clogged pore ruptures beneath the skin, spilling its contents (oil, bacteria, dead cells, immune cells) into the surrounding tissue. Your body walls off this material by forming a membrane around it, creating a fluid-filled sac deep under the skin. That sac is the cyst, and it’s filled with pus.
Nodular acne looks similar from the outside but feels harder. Nodules are more solid than cysts because they contain less fluid and more dense, inflamed tissue. Both types form well below the surface, which is why they rarely come to a visible head and can linger for weeks. The deeper location also means more pressure on nerves and a higher risk of scarring, since the inflammation damages the structural layer of skin rather than just the surface.
The Full Ingredient List
Putting it all together, a typical inflamed pimple contains:
- Sebum: a mix of triglycerides, squalene, wax esters, and cholesterol produced by your oil glands
- Keratin and dead keratinocytes: the protein-rich skin cells that formed the original plug
- Cutibacterium acnes: bacteria (both living and dead) that colonized the blocked pore
- Bacterial byproducts: propionate, porphyrins, and free fatty acids released as the bacteria feed on oil
- Neutrophils and macrophages: white blood cells sent to fight the infection
- Inflammatory molecules: cytokines and other signaling chemicals that cause redness, swelling, and pain
- Tissue fluid: liquid that seeps in from surrounding blood vessels as part of the inflammatory response
A blackhead contains only the first two items on that list. A whitehead adds a thin skin covering. A pustule (the classic “zit” with a white head) contains all of them. And a cyst is that entire mixture walled off deep under the skin, often under enough pressure that it feels like a marble beneath your fingertip.