Cysts under the skin form when skin cells get trapped beneath the surface and create a small sac that fills with a protein called keratin, the same material that makes up your hair and the outer layer of your skin. The result is a slow-growing, round bump that feels firm and can range from pea-sized to several centimeters across. Most skin cysts are harmless, but understanding what causes them helps you recognize what you’re dealing with and whether it needs attention.
How Skin Cells Get Trapped
Your skin constantly sheds dead cells from its outer layer. Normally these cells flake off and disappear. But sometimes, instead of shedding outward, cells migrate deeper into the skin. Once buried, they continue doing what skin cells do: producing keratin. With nowhere for that keratin to go, it accumulates inside a pocket lined by those misplaced cells. Over weeks or months, the pocket grows into a visible lump.
This process most often begins in the upper part of a hair follicle. Damage, inflammation, or a clogged pore at the follicle’s opening can redirect surface cells inward. The lining of the resulting cyst is identical to the lining of the hair follicle it came from, which is why these cysts tend to appear in areas with dense hair follicles: the face, neck, chest, upper back, and scalp.
Blocked Follicles and Glands
A blocked hair follicle is the single most common trigger. When oil, dead skin, or debris plugs the follicle opening, the cells and secretions that would normally reach the surface get trapped underneath. The follicle essentially turns inside out, forming a closed sac. This is why people sometimes notice a tiny dark dot (a plugged pore) at the center of a cyst.
Oil glands themselves can also become obstructed. On the eyelids, for example, blocked oil glands produce chalazia, firm bumps that form when trapped secretions leak into surrounding tissue and trigger an inflammatory reaction. Similar blockages in oil glands elsewhere on the body can seed cyst formation, though these are less common than follicle-based cysts.
Injury and Trauma
On parts of your body that don’t have many hair follicles, cysts can still form after a cut, scrape, surgical incision, or other skin injury. The trauma physically pushes surface skin cells below the top layer into the deeper layer called the dermis. Once lodged there, these displaced cells create a pocket where keratin collects and a cyst gradually develops. This type is sometimes called an implantation cyst, and it can show up on the palms, soles, or other areas where follicle-driven cysts are rare.
Even minor, repeated friction or irritation can have the same effect over time. Tight clothing, regular shaving, or chronic rubbing against equipment can damage follicles enough to set the process in motion.
Genetic Factors
Some people are simply more prone to cysts than others, and genetics play a real role. If your parents developed skin cysts, your chances are higher. A few specific genetic conditions make cysts especially common. Steatocystoma multiplex, caused by a mutation in a gene involved in skin structure (KRT17), leads to dozens or even hundreds of small cysts that originate in oil glands and fill with an oily substance called sebum rather than keratin. This condition is closely related to another inherited skin disorder called pachyonychia congenita, which involves the same gene.
Gardner syndrome, a rare inherited condition, also causes multiple skin cysts alongside other growths. People who develop cysts repeatedly or in unusual numbers may have an underlying genetic predisposition worth investigating.
Types of Cysts and Where They Appear
Not all skin cysts are identical. The two most common types, epidermoid and pilar cysts, have historically been lumped together as “sebaceous cysts.” That term is misleading because neither one actually arises from oil-producing sebaceous gland cells. The difference between them comes down to the type of cell lining the sac wall.
- Epidermoid cysts are lined with cells from the skin’s outer layer. They’re most common on the face, neck, chest, upper back, and sometimes the groin. The contents look like thick, white toothpaste.
- Pilar cysts are lined with cells resembling those found in hair roots. They appear almost exclusively on the scalp and tend to run in families. Their contents are similar but slightly denser.
- Dermoid cysts are present from birth, though they may not become noticeable until later. They typically appear near the eyebrows, on the scalp, over the collarbone, or on the chest.
Both epidermoid and pilar cysts contain that same cheesy, keratin-rich material. They grow slowly, usually over months to years, and can stay small indefinitely or gradually enlarge.
Cysts vs. Other Lumps
A firm, round bump under the skin isn’t always a cyst. Lipomas, the most common soft-tissue lumps, feel soft and rubbery rather than firm. They move easily under your fingers and tend to show up on the shoulders, neck, stomach, and back. Unlike cysts, lipomas are made of fat cells, not keratin, and they almost never become painful or inflamed.
An abscess, by contrast, is an active infection. It develops quickly (days, not months), feels warm and tender, and the overlying skin is usually red and swollen. Abscesses are filled with pus rather than keratin and generally need drainage or antibiotics. If a lump appears suddenly and hurts, it’s more likely an abscess or an inflamed cyst than a simple cyst growing on its own timeline.
Ganglion cysts are a different category entirely. These fluid-filled lumps form on joints or tendons, most often on the wrist or hand, and contain a thick, jelly-like fluid rather than keratin.
What Happens When a Cyst Gets Inflamed
A cyst that’s been sitting quietly for months or years can suddenly become red, swollen, and painful. This usually happens when the sac wall ruptures internally, spilling keratin into the surrounding tissue. Your immune system treats that leaked keratin as a foreign substance and mounts an inflammatory response. The cyst swells, the skin over it turns red, and it can become quite tender.
This inflammatory flare is not the same as a bacterial infection, though the two can look similar. A true infection involves bacteria entering the cyst, which can happen if the skin over it breaks or if you try to squeeze or pop it. Infected cysts may produce actual pus, develop increasing redness that spreads outward, and sometimes cause fever.
Why Squeezing Makes Things Worse
The urge to squeeze a visible cyst is understandable, but it reliably backfires. Pressing on a cyst can rupture its internal wall, triggering the inflammatory reaction described above. Even if you manage to express some of the contents through the skin’s surface, the sac lining remains intact beneath the skin. As long as that lining exists, it will continue producing keratin and the cyst will refill, often within weeks.
Attempting to puncture or cut open a cyst at home introduces bacteria directly into the pocket, dramatically increasing the risk of infection. It can also push the cyst contents deeper into surrounding tissue, worsening inflammation and potentially causing scarring. If a cyst bothers you cosmetically or keeps getting inflamed, a minor in-office procedure to remove the entire sac, including its lining, is the only way to prevent it from coming back.