Cysts form when cells, fluid, or debris get trapped beneath the surface of tissue and the body builds a sac around them. The specific trigger depends on the type of cyst, but the basic process is the same: something that should have been shed, released, or drained gets sealed off instead, and a closed pocket develops. Some cysts are caused by a blocked hair follicle, others by a normal hormonal cycle that didn’t finish its job, and still others by physical friction pushing hair into skin where it doesn’t belong.
How Skin Cysts Form
The most common type of cyst is the epidermoid cyst, which accounts for roughly 80 to 90 percent of all skin cysts. Your skin constantly sheds dead cells from its outer layer. Normally those cells flake off and disappear. But sometimes they migrate deeper into the skin instead of shedding. When that happens, the misplaced cells form a tiny walled-off sac and begin secreting keratin, a thick protein your body also uses to build hair and nails. That keratin accumulates inside the sac as a dense, cheese-like substance, and the lump slowly grows over weeks or months.
Skin injury or irritation is one of the most common triggers. A scratch, a surgical wound, or damage to a hair follicle can push surface cells beneath the skin where they don’t belong. Once trapped, the body’s inflammatory response wraps those misplaced cells in tissue, sealing them off and creating the cyst structure. Epidermoid cysts are most common between the ages of 20 and 50, though tiny versions called milia frequently appear in newborns and usually resolve on their own.
Chronic skin conditions like acne or hidradenitis suppurativa (a condition that causes recurring inflamed lumps) also increase your chances of developing cysts, because ongoing inflammation and blocked pores create more opportunities for cells to get trapped.
Pilonidal Cysts: Friction and Ingrown Hair
Pilonidal cysts develop near the tailbone when loose hairs puncture the skin and become embedded. Friction and pressure drive the process. Tight clothing, prolonged sitting, and cycling can all force stray hairs into the skin at the base of the spine. Once the hair is lodged beneath the surface, the body treats it as a foreign object and builds a cyst around it in an attempt to push it out.
These cysts are especially common in younger men and people who sit for long stretches. If bacteria enter the cyst, it can become infected and fill with pus, turning painful and sometimes requiring drainage.
Ovarian Cysts and the Menstrual Cycle
Most ovarian cysts aren’t caused by anything going wrong. They’re a byproduct of a normal menstrual cycle. Each month your ovaries grow small fluid-filled sacs called follicles that produce hormones and eventually release an egg. When this process doesn’t complete as expected, a cyst can form in one of two ways.
A follicular cyst develops when the follicle doesn’t rupture to release its egg. Instead of breaking open at ovulation, it keeps growing and fills with fluid. A corpus luteum cyst forms after the egg has been released: the opening where the egg exited seals shut, and fluid accumulates inside the now-closed follicle. Both types are called functional cysts, and most resolve on their own within a few menstrual cycles without treatment.
Fertility medications that stimulate ovulation can increase the risk of these cysts, because they push the ovaries to produce more follicles than usual.
Ganglion Cysts Near Joints and Tendons
Ganglion cysts are firm, round lumps that typically appear on the wrist, hand, or foot. They form when the tissue covering a joint or tendon develops a small tear. That damaged tissue bulges outward and creates a sac that fills with a thick, jelly-like fluid similar to the lubricating fluid normally found inside joints. A prior injury to the area may be involved, though many ganglion cysts appear without any obvious cause. They’re not cancerous and sometimes disappear without treatment, but they can press on nearby nerves and cause discomfort.
Genetic Conditions That Cause Multiple Cysts
Some people develop cysts repeatedly because of inherited conditions. Gardner syndrome, a rare genetic disorder, predisposes people to skin cysts along with hundreds of colon polyps and other growths throughout the body. Research has also identified specific genetic disruptions that cause cysts: when certain proteins responsible for holding skin cells together are missing or dysfunctional, cells can separate from surrounding tissue, become trapped, and form cysts in and around hair follicles. These genetic causes are uncommon but worth considering if you or a family member develops cysts frequently in multiple locations.
How a Cyst Differs From an Abscess
A cyst is a closed sac, usually round and firm, sitting just beneath the skin. It grows slowly, isn’t contagious, and is almost never cancerous. It may have a small dark spot in the center. An abscess, by contrast, is a pocket of pus caused by bacterial infection. It forms when white blood cells rush to an infected area, accumulate, and create a soft, warm, red mass that’s tender to the touch. The pus inside an abscess is highly contagious. If you’re unsure which you’re dealing with, the texture is a useful clue: cysts feel firm and relatively painless, while abscesses are soft, warm, and hurt.
When a Cyst Becomes Dangerous
Most cysts are harmless. The situations that change this are infection, rupture, and (for ovarian cysts) torsion. An infected cyst fills with pus and becomes painful, red, and swollen. A ruptured ovarian cyst can cause sharp abdominal pain. If an infected ovarian cyst ruptures, it can trigger sepsis, a potentially life-threatening immune response to bacteria spreading through the body. Ovarian torsion occurs when the weight of a large cyst causes the ovary to twist, cutting off its blood supply.
Seek emergency care for abdominal pain paired with severe nausea and vomiting, fever, heavy vaginal bleeding, or faintness. These can signal torsion, infection, or significant internal bleeding from a ruptured cyst.
Reducing Your Risk
There’s no guaranteed way to prevent cysts, but a few habits can lower your chances. Keeping skin clean and gently exfoliated helps prevent dead cells from building up around hair follicles. Avoiding tight clothing in friction-prone areas, especially the tailbone region, reduces the mechanical force that drives hair into skin. Managing chronic skin conditions like acne with a dermatologist limits the ongoing inflammation that makes cysts more likely.
If you already have a cyst, never squeeze or pop it. Forcing the contents out can rupture the sac deeper into the tissue, causing severe inflammation or infection. A dermatologist can safely drain or remove a cyst with the sac intact, which is the only way to reduce the chance of it coming back in the same spot.