Sebaceous cysts form when dead skin cells get trapped beneath the surface of your skin instead of shedding normally. Rather than flaking off as part of your skin’s natural turnover cycle, these cells collect in a pocket under the skin and fill with a protein called keratin, the same tough material that makes up your hair and nails. Over time, that pocket grows into a firm, round lump you can feel and sometimes see.
Despite the name, most “sebaceous cysts” don’t actually involve the oil-producing glands in your skin at all. The medical community now calls them epidermal inclusion cysts or epidermoid cysts, because the cyst wall is made of regular skin cells and the contents are compacted keratin, not oil (sebum). The old name stuck in everyday language, but understanding what’s really inside helps explain why they form and how they behave.
How a Cyst Develops Step by Step
Your skin constantly produces new cells at its deepest layer and pushes older ones toward the surface, where they eventually die and fall off. This process is seamless most of the time. A cyst begins when something disrupts that outward path, forcing skin cells inward instead.
The most common starting point is a blockage at the top of a hair follicle, the tiny opening where a strand of hair exits the skin. When that opening gets plugged, the dead cells that would normally travel up and out have nowhere to go. They accumulate just below the surface, forming a sac lined with skin tissue. The sac steadily fills with layers of keratin, which gives the cyst its characteristic thick, yellowish-white contents and its sometimes unpleasant smell when ruptured.
On parts of the body without hair follicles, the process works a little differently. A cut, scrape, surgical incision, or other injury can push surface skin cells down into the deeper layer of skin (the dermis). Once embedded there, those displaced cells continue to behave like surface cells, producing keratin and building up inside a pocket that becomes a cyst. This is why cysts sometimes appear at the site of an old wound or along a surgical scar.
What Causes the Blockage in the First Place
Several things can set off the chain of events that leads to a cyst:
- Skin injuries. Scratches, puncture wounds, and surgical cuts can all push skin cells beneath the surface or damage a hair follicle enough to block it.
- Acne. Chronic acne involves repeated inflammation and plugging of follicles, creating the exact conditions that trap skin cells.
- Chronic sun damage. Long-term UV exposure changes the structure of skin cells and can disrupt normal shedding, making blockages more likely.
- Skin irritation. Friction from clothing, shaving, or repeated rubbing can injure follicles at a microscopic level.
Sometimes, though, a cyst appears with no obvious trigger. The follicle simply clogs for reasons that aren’t clear, the same way a pore can become blocked and cause a pimple, except the blockage happens deeper and the resulting pocket is more durable.
Who Gets Them
Anyone past puberty can develop an epidermoid cyst. They’re rare in children, likely because the hormonal changes of puberty increase oil production and skin cell turnover, creating more opportunities for follicles to clog. People with a history of acne or frequent skin injuries are at higher risk.
There is also a genetic component. A rare inherited condition called Gardner syndrome causes people to develop multiple cysts, often alongside other growths. For most people, though, a single cyst is a one-off event or an occasional recurrence rather than a sign of a broader condition.
Where They Tend to Show Up
Cysts are most common on the face, neck, upper back, and trunk, areas dense with hair follicles. The scalp is another frequent location. Because follicle blockage is the primary mechanism, areas with more follicles simply have more opportunities for things to go wrong. That said, cysts can appear almost anywhere on the body, including the hands, feet, and other spots with few or no follicles, if a skin injury pushes cells beneath the surface.
How to Tell It’s a Cyst
A typical cyst feels like a smooth, firm, round bump just under the skin. It moves freely when you press on it, unlike a deeper growth that might feel anchored in place. Many cysts have a tiny dark dot at the center called a punctum, which is the blocked opening of the original follicle. They usually grow slowly over months or years and are painless unless they become inflamed.
Inflammation is a common source of confusion. When a cyst’s wall ruptures internally, its keratin contents leak into the surrounding tissue and trigger a strong inflammatory reaction. The area turns red, swollen, and painful, which looks a lot like an infection. Doctors sometimes mistake an inflamed cyst for an abscess and prescribe antibiotics, but the redness is usually a chemical irritation response rather than a bacterial infection.
Why They Come Back
Squeezing or draining a cyst at home might temporarily flatten it, but it almost always returns. That’s because the cyst is a self-contained sac with its own lining. As long as that lining remains in your skin, it will continue producing keratin and refilling. Even a clinical drainage procedure (incision and drainage) has a high recurrence rate for the same reason.
Complete removal requires cutting out the entire sac wall, which a dermatologist or surgeon does as a minor outpatient procedure. When the full wall is removed, the cyst has no structure left to regenerate.
Reducing Your Risk
Because most cysts start with follicle blockages or minor skin injuries, a few practical habits can lower your chances. Keeping your skin clean and gently exfoliated helps dead cells shed on their own rather than accumulating in follicles. Treating acne early reduces the repeated follicle inflammation that sets up cyst formation. Protecting your skin from chronic sun damage with sunscreen or clothing removes another known trigger. And if you do get a cut or scrape, keeping the wound clean and letting it heal properly gives skin cells the best chance of staying where they belong, on the surface, rather than getting pushed deeper where they can start building a cyst.