Most skin cysts form when surface skin cells get trapped beneath the skin’s outer layer, creating a small pocket that fills with a protein called keratin. This is the single most common cause, but the specific trigger varies: a blocked hair follicle, a cut or scrape, sun damage, genetics, or chronic inflammation can all set the process in motion. The type of cyst you develop depends on where in the skin it forms and what fills it.
Blocked Hair Follicles: The Most Common Cause
The majority of skin cysts are epidermoid cysts (sometimes incorrectly called sebaceous cysts), and they start at the top of a hair follicle. When the opening of a follicle becomes blocked, irritated, or inflamed, skin cells that would normally shed from the surface get pushed inward instead. Those cells settle into the deeper layer of skin called the dermis, where they form a closed sac. The sac keeps producing keratin, the same tough protein that makes up your hair and nails, and that material accumulates with nowhere to go. The result is a firm, round lump just under the skin that grows slowly over weeks or months.
Despite the popular name “sebaceous cyst,” most of these lumps are not filled with oil. They’re packed with keratin and dead cell debris. True sebaceous cysts, filled with the oily substance your skin produces, are actually rare and associated with a specific condition called steatocystoma multiplex.
Skin Injuries and Surgery
On parts of the body where you don’t have many hair follicles, cysts can still form after physical trauma. A cut, puncture wound, scrape, or surgical incision can push surface skin cells below the outer layer and into the dermis. Once those cells are embedded deeper than they belong, they create a pocket and begin accumulating keratin, just like a follicle-based cyst. This is sometimes called an implantation cyst, and it can appear weeks to months after the original injury, long after the wound itself has healed.
This mechanism explains why some people notice a cyst at the site of an old scar, piercing, or surgical incision. The trauma doesn’t need to be dramatic. Even repeated friction or minor skin irritation can displace enough cells to start the process.
Sun Damage and Aging
Older adults, particularly those with years of sun exposure, develop skin cysts more frequently. Chronic sun damage thins the outer layer of skin and makes it more fragile. This fragility disrupts hair follicles, causing them to become plugged with excess keratin. The plugged follicles can then develop into open comedones (large blackheads) or, eventually, cysts. If you’ve noticed small firm bumps appearing on sun-exposed areas like the face, neck, or forearms as you’ve gotten older, accumulated UV damage to the skin’s structure is a likely contributor.
Genetics and Inherited Conditions
Some people are simply more prone to skin cysts because of their genetic makeup. Pilar cysts, which originate from the outer root sheath of a hair follicle rather than the top, are a clear example. These firm, smooth lumps appear most often on the scalp and follow an autosomal dominant inheritance pattern, meaning if one of your parents had them, you have roughly a 50% chance of developing them too.
Certain genetic syndromes also predispose people to multiple skin cysts. Gardner syndrome, caused by a mutation in a tumor-suppressing gene called APC, leads to growths throughout the body, including cysts under the skin. If you develop many cysts at a young age, especially alongside other unusual growths, an underlying genetic condition may be the reason.
Acne and Chronic Inflammation
Severe acne is one of the more common inflammatory triggers for skin cysts. In acne, excess keratin plugs the hair follicle while oil builds up behind the blockage. If the clogged pore swells enough, it can rupture beneath the skin’s surface, triggering an intense inflammatory response. Certain strains of skin bacteria amplify this inflammation, and an imbalance in the skin’s overall bacterial community makes it worse. Over time, this repeated cycle of clogging, rupturing, and inflaming can produce deep, cyst-like nodules that persist for weeks.
In its most severe form, called acne conglobata, these interconnected cysts and abscesses can merge under the skin, creating large painful masses. This goes well beyond typical breakouts. It represents a sustained inflammatory process where the skin’s normal shedding, oil production, and bacterial balance have all gone off track simultaneously.
Other Types of Skin Cysts
Not every skin cyst comes from trapped keratin. A few other types have distinct causes worth knowing about:
- Digital mucous cysts appear on fingers or toes, usually between the last joint and the nail. They form when the lining of a joint grows excessively, creating a small sac connected to the joint by a stalk. Osteoarthritis is the primary driver. Between 64% and 93% of people with osteoarthritis develop these cysts.
- Ganglion cysts form near joints or tendons, most often on the wrist. They fill with a thick, jelly-like fluid from the joint capsule rather than keratin.
- HPV-related cysts can develop in a small number of cases when certain strains of human papillomavirus trigger abnormal cell behavior in the skin, leading to cyst formation.
How to Tell a Cyst From Something Else
A typical skin cyst is round, firm, and sits just under the surface. It grows slowly, sometimes over months or years, and is usually painless unless it becomes inflamed. You may notice a small dark dot in the center, which is the blocked pore opening. It moves slightly when you press on it.
An abscess looks different. It’s usually pink or red, warm to the touch, soft rather than firm, and painful. An abscess is a pocket of pus caused by bacterial infection, not a slow buildup of protein. If a previously painless cyst suddenly becomes red, swollen, and tender, it may have ruptured internally or become infected. When a cyst wall breaks, the keratin inside leaks into surrounding tissue, triggering a sharp inflammatory reaction that can mimic infection even when no bacteria are involved.
Why Cysts Come Back
If you’ve had a cyst drained or squeezed and it returned in the same spot, that’s because the sac wall was left behind. A skin cyst is like a small bag embedded in your skin. Draining it empties the contents, but the bag itself remains and will refill over time. Complete removal requires taking out the entire cyst wall, which is why cysts treated with simple drainage or compression have a high recurrence rate. For cysts that bother you cosmetically or keep getting inflamed, surgical excision of the full sac is the only reliable way to prevent them from coming back.