Skin cysts form when skin cells that would normally shed from your body’s surface instead get trapped beneath it. These displaced cells create a small pocket, or sac, that fills with a protein called keratin, the same tough material that makes up your hair and nails. The result is a slow-growing, round lump just under the skin that’s almost always harmless but can stick around for years if left alone.
How Skin Cells Get Trapped
Your skin’s outer layer constantly produces new cells and sheds old ones. Normally, those aging cells flake off without you noticing. A cyst starts when something disrupts that process and pushes surface cells deeper into the skin instead of outward. Once buried, those cells don’t just die off. They organize into a tiny wall, essentially rebuilding a miniature version of the skin’s surface underground, and begin pumping keratin into the enclosed space. That keratin accumulates over weeks or months, and the lump slowly grows.
Most of the time, this happens at a hair follicle. The opening at the top of the follicle gets damaged, inflamed, or blocked, and the cells lining it start building inward rather than growing outward. That’s why cysts are most common on the face, neck, and trunk, areas dense with hair follicles. On parts of the body without hair follicles, like the palms and soles, cysts can still form but usually only after a direct injury pushes skin cells below the surface.
Common Triggers
Several everyday situations can set this process in motion:
- Skin injuries. A cut, scrape, surgical incision, or even a splinter can drive surface cells into the deeper layer of skin (the dermis). Once implanted there, those cells can form a cyst wall and start collecting keratin.
- Acne and chronic inflammation. Ongoing irritation around hair follicles, whether from acne, repeated friction, or chronic sun damage, can block follicle openings and redirect cell growth inward.
- Follicle damage. Anything that disrupts a hair follicle, from shaving nicks to ingrown hairs, can trigger cyst formation at the top of the follicle where it meets the skin surface.
In a small number of cases, certain strains of HPV (human papillomavirus) have been linked to cyst development, particularly on the hands and feet. And rarely, cysts form before birth when clusters of skin cells get sealed off during embryonic development, only becoming noticeable later in life.
The “Sebaceous Cyst” Misunderstanding
You’ve probably heard the term “sebaceous cyst,” but it’s mostly a misnomer. The vast majority of lumps called sebaceous cysts are actually epidermal cysts filled with keratin, not with the oily substance (sebum) your skin glands produce. Sebum-filled cysts do exist, but they’re tied to a specific condition called steatocystoma multiplex, which causes dozens of small cysts to develop across the chest, arms, and other areas. True sebaceous cysts are uncommon compared to the keratin-filled kind.
The distinction matters because it points to different underlying causes. A standard epidermal cyst comes from trapped skin cells. A true sebaceous cyst comes from an abnormality in the oil gland itself, often driven by a genetic mutation.
Genetic and Hormonal Factors
Some people are genetically predisposed to developing multiple cysts. Steatocystoma multiplex, the condition that produces true sebum-filled cysts, is caused by mutations in the KRT17 gene, which provides instructions for building a structural protein in skin and hair. The condition runs in families in an autosomal dominant pattern, meaning inheriting one copy of the mutated gene from either parent is enough to cause it. A related condition called pachyonychia congenita involves the same gene but also produces severe nail abnormalities and painful thickening of skin on the palms and soles.
Hormones play a role, too. Steatocystoma multiplex typically first appears during puberty, when testosterone levels peak and ramp up oil production in the skin’s glands. Research on transgender men starting testosterone therapy has shown that the hormone can trigger rapid proliferation of these cysts, reinforcing the link between androgens and sebaceous gland activity. For ordinary epidermal cysts, the hormonal connection is less direct, but the fact that they’re more common after puberty suggests that increased oil production and follicle activity create more opportunities for blockages.
Why Some Cysts Become Painful
Most skin cysts sit quietly under the surface for months or years without causing problems. They become painful when the cyst wall ruptures, either from pressure, a bump, or from squeezing it. When that wall breaks, keratin leaks into the surrounding tissue. Your immune system treats this leaked protein as a foreign invader, launching an inflammatory response that causes redness, swelling, warmth, and tenderness. This is often mistaken for an infection, but it’s usually a sterile inflammation, your body reacting to its own displaced material.
True infection can happen, though, especially if bacteria enter through a break in the skin over the cyst. An infected cyst becomes hot, very tender, and may drain pus. This is one reason squeezing or popping a cyst at home tends to make things worse rather than better.
Why They Come Back
A cyst can be drained, but if the sac wall remains under the skin, it will refill. That’s the core reason cysts recur. Complete removal of the entire cyst wall through a minor surgical procedure is the only reliable way to prevent regrowth. When the full wall is removed, recurrence rates are low, around 3% regardless of the specific surgical technique used. Simple drainage without removing the wall virtually guarantees the cyst will return.
This also explains why home remedies and warm compresses, while they can reduce inflammation temporarily, don’t make a cyst disappear permanently. The sac is a self-contained structure. As long as it’s intact, it will keep producing keratin and slowly refilling.
Who Gets Them Most Often
Skin cysts can develop at any age, but they’re most common in adults. They rarely appear before puberty unless linked to a genetic condition. People with a history of acne, frequent skin injuries, or chronic sun damage face a higher risk simply because those conditions create more opportunities for follicle disruption and cell implantation. Having one cyst doesn’t necessarily mean you’ll get more, but some people do seem prone to developing them repeatedly in different locations, likely due to individual differences in how their skin sheds and regenerates cells.