Epidermoid cysts form when skin cells that normally shed from your body’s surface instead move deeper into the skin and keep multiplying. These displaced cells create a small pocket lined with skin tissue, which then fills with keratin, a thick, yellowish protein your skin naturally produces. The result is a firm, round bump just beneath the surface that grows slowly over time.
How Skin Cells Get Trapped
Your outer layer of skin constantly sheds old cells and replaces them with new ones. Normally, these cells work their way outward and fall off. But sometimes the process reverses: cells migrate inward, settling into the deeper layer of skin called the dermis. Once there, they behave as if they’re still on the surface, forming a tiny walled-off sac and continuing to produce keratin. Because the keratin has nowhere to go, it accumulates inside the sac, and the cyst gradually expands.
This is why epidermoid cysts feel doughy or firm to the touch and sometimes release a thick, cheese-like substance if they rupture. That material is compressed keratin, not pus (unless the cyst has become infected).
Skin Injuries and Irritation
Physical damage to the skin is one of the clearest triggers. A scratch, surgical wound, or penetrating injury can push surface cells into deeper tissue, essentially planting the seed for a cyst. Acne is another common cause: repeated inflammation around a pore can disrupt the normal path skin cells take as they shed. Chronic sun damage does the same thing over time, altering the skin’s structure enough to trap cells beneath the surface.
On parts of the body without hair follicles, such as the palms or soles, cysts almost always trace back to some form of trauma or injury. In hair-bearing areas, the mechanism is slightly different.
Hair Follicle Damage
Hair follicles are one of the most common starting points for epidermoid cysts. Each follicle is a tiny tunnel lined with the same type of cells that cover your skin’s surface. If that tunnel gets blocked, inflamed, or ruptured, the lining cells can end up trapped in the surrounding tissue. From there, they form a cyst wall and start filling with keratin, just as they would on the surface.
This explains why epidermoid cysts so often appear on the face, neck, scalp, and trunk, all areas dense with hair follicles. Anything that chronically irritates follicles (shaving, friction from clothing, recurring acne) can increase the likelihood of a cyst forming.
Who Gets Them Most Often
Epidermoid cysts can show up at any age, but they’re rare before puberty. A large review of 432 cases found they peak in the 40s and 50s, with each decade accounting for about 18% of cases. People in their 20s and 30s made up the next largest groups. Men develop them more often than women, though the reported ratio varies widely across studies, from roughly 1.5 men per woman to as high as 4.6 to 1.
Genetic Conditions That Cause Multiple Cysts
A single epidermoid cyst is almost never a sign of a genetic problem. Multiple cysts appearing together, especially at a young age, can occasionally point to an inherited condition called Gardner syndrome. This is a rare disorder (roughly 1 in 7,000 to 1 in 30,000 births) caused by a mutation on chromosome 5 that follows an autosomal dominant pattern, meaning a child needs only one copy of the gene from one parent to be affected.
Gardner syndrome is primarily known for causing polyps in the colon, but it also produces a constellation of other growths: bone tumors in the jaw and skull, soft tissue masses, dental abnormalities, and epidermoid cysts. The cysts themselves aren’t dangerous, but their presence alongside other unusual growths can be an early clue that prompts screening for the more serious intestinal features.
The Role of HPV
A small number of epidermoid cysts have been linked to human papillomavirus infection. The strains most commonly identified in cyst tissue are HPV types 57 and 60, and these cases tend to occur on the palms and soles. Isolated reports have also found HPV types 6 and 11 in cysts on other body sites. This connection is uncommon enough that routine HPV testing isn’t part of cyst evaluation, but it does suggest that viral infection can sometimes trigger the same process of trapping skin cells beneath the surface.
When a Cyst Becomes Inflamed
Epidermoid cysts often sit quietly under the skin for months or years. Problems start when the cyst wall ruptures, either from pressure, trauma, or attempted squeezing. When keratin leaks into the surrounding tissue, your immune system treats it as a foreign substance and mounts an inflammatory response. The area becomes red, swollen, warm, and painful. This is frequently mistaken for infection, but it’s actually a reaction to the keratin itself. True bacterial infection can occur on top of that inflammation, especially if the skin over the cyst has been broken.
Epidermoid Cysts vs. “Sebaceous Cysts”
If you’ve heard the term “sebaceous cyst,” it almost certainly referred to an epidermoid cyst. The older name has largely been abandoned because it’s misleading: these cysts contain no oil-producing (sebaceous) glands. The lining is made entirely of the same layered skin cells found on your body’s surface, and the contents are keratin, not sebum. True sebaceous cysts do exist but are far less common. If a doctor or search result uses the term “sebaceous cyst,” they’re most likely describing an epidermoid cyst.
Risk of Cancer
Malignant transformation of an epidermoid cyst into squamous cell carcinoma is exceedingly rare, with reported rates between 0.011% and 0.045%. In practical terms, this means the overwhelming majority of epidermoid cysts are completely benign. A cyst that suddenly grows rapidly, becomes fixed to deeper tissue, or changes in character after being stable for years warrants evaluation, but the baseline risk is very low.