Cysts form when fluid, air, or semi-solid material accumulates inside a closed sac of tissue, and the body has no natural drainage pathway to clear it. The specific trigger depends on location: a blocked duct, a hormonal signal gone wrong, joint stress pushing fluid where it doesn’t belong, or a genetic mutation that causes cells to multiply and trap fluid. Despite their variety, nearly all cysts share one basic feature: a lining of cells that walls off the contents from surrounding tissue.
Blocked Ducts and Trapped Skin Cells
The most common cysts people notice on their skin, often called sebaceous cysts, are technically epidermoid cysts. They form when a hair follicle’s duct becomes obstructed, trapping the skin protein keratin beneath the surface. This can happen after acne ruptures a follicle, after a minor skin injury pushes surface cells deeper into tissue, or because of a developmental defect in the duct itself. Once trapped, keratin keeps accumulating inside the sealed pocket, and the cyst slowly enlarges.
A related type, the pilar cyst, occurs almost exclusively on the scalp. These cysts are lined by the same type of skin cells found in the outer sheath of a hair follicle. Those cells produce keratin rapidly and without the usual transitional layer that normal skin goes through, so the cyst fills with a dense, homogenous plug of keratin rather than the cheesy material typical of epidermoid cysts. The result is a firm, smooth bump that can persist for years without symptoms unless it becomes inflamed.
Hormonal Disruptions in the Ovaries
Functional ovarian cysts are the most common type in premenopausal women, and they arise directly from the normal menstrual cycle. Each month, a follicle in the ovary matures and releases an egg. If the hormonal signals that trigger egg release are disrupted, the follicle keeps growing and fills with fluid instead of rupturing. These follicular cysts are thin-walled and produce estrogen while progesterone stays low (below 1 ng/mL in clinical measurements).
A second type, the corpus luteum cyst, forms after the egg has been released. The empty follicle normally shrinks and dissolves, but sometimes it seals shut and fluid accumulates inside. These cysts have thicker walls (over 3 mm) and produce higher levels of progesterone. Most functional cysts of either type resolve on their own within one to three menstrual cycles. When a corpus luteum cyst ruptures, though, it can cause sudden lower abdominal pain, nausea, dizziness, and internal bleeding that occasionally requires surgical intervention.
Joint Stress and Ganglion Cysts
Ganglion cysts, the rubbery bumps that appear near wrists and finger joints, form through a different process entirely. The leading theory is that repetitive or acute stress on a joint creates a small tear in the joint capsule. Synovial fluid, the lubricant inside every joint, leaks through that tear into surrounding tissue. The leaked fluid reacts with local tissue, producing a thick, gelatinous material that becomes walled off in a cyst.
Pre-existing joint problems make this more likely. Ligament injuries near the wrist, for instance, alter the joint’s mechanics, gradually weaken the capsule, and eventually allow fluid to escape. An alternative explanation is that joint stress triggers nearby connective tissue to degenerate into a mucus-like substance that pools and forms a cyst even without a capsule tear. Either way, the common thread is mechanical wear and the body’s attempt to contain displaced fluid.
Genetic Mutations That Drive Cyst Growth
Some cysts are genetically programmed. Autosomal dominant polycystic kidney disease (ADPKD), one of the most common inherited disorders, causes hundreds of fluid-filled cysts to develop in both kidneys over decades. It results from mutations in one of two genes. The first, responsible for roughly 85% of cases, produces a large protein embedded in cell membranes. The second, accounting for about 15%, produces a protein that functions as a calcium channel. Together, these proteins form a sensor on the hair-like projections (cilia) of kidney tubule cells.
When both copies of the gene lose enough function to drop below a critical threshold, the affected cell loses its ability to regulate growth. It begins multiplying, forming an outpouch from the kidney tubule lined by a single layer of mutant cells. Over time, this pouch detaches from the tubule entirely, becoming an isolated cyst that continues to expand as the cells lining it secrete fluid inward. The loss of calcium signaling also overactivates a chemical messenger inside cells that further drives fluid secretion and tissue scarring. This is why ADPKD cysts don’t just form once and stop; they progressively enlarge and multiply.
The Immune System’s Walling-Off Response
Your body also creates cyst-like structures as a defense mechanism. When the immune system encounters something it can’t break down or remove, such as a foreign object, a splinter, or certain types of surgical material, it switches to a containment strategy. Immune cells called macrophages arrive first and attempt to digest the material. When that fails, they fuse together into giant cells that persist at the site, sustaining a low-grade inflammatory state.
These giant cells then recruit other cells that produce collagen, the structural protein in scar tissue. Within two to four weeks, a dense, largely bloodless capsule of collagen forms around the foreign material, effectively sealing it off from the rest of the body. This fibrotic capsule functions much like a cyst wall. The inflammation settles once encapsulation is complete, but the capsule itself remains indefinitely. The same principle applies to certain parasitic infections: when the body can’t kill the organism, it walls it off in a calcified cyst instead.
Cysts vs. Abscesses and Solid Growths
Not every lump is a cyst. The distinction matters because treatment differs significantly. A cyst grows slowly and is usually painless unless it presses against a nerve or becomes large enough to interfere with nearby structures. An abscess, by contrast, is a pocket of pus caused by active infection. It’s red, swollen, warm, and often intensely painful. If you notice redness and heat around a lump that appeared quickly, infection is the more likely explanation.
Telling a cyst from a solid tumor can be trickier. On ultrasound, cysts appear as dark, fluid-filled spaces with smooth borders and a bright signal behind them (called posterior enhancement). Solid tumors tend to show internal blood flow on Doppler imaging. However, some solid tumors mimic cysts closely: in one radiologic analysis of 23 cyst-like musculoskeletal masses, 7 appeared completely fluid-filled and 10 showed no detectable blood flow, yet all turned out to be solid. This is why imaging that looks straightforward can still warrant further evaluation, particularly for lumps that are growing, changing shape, or appearing in unusual locations.
Why Some Cysts Recur
Cysts often come back after drainage because the sac lining remains intact. Draining an epidermoid cyst with a needle removes the contents but leaves the keratin-producing wall in place, and it simply refills. Complete removal of the cyst wall is what prevents recurrence. The same applies to ganglion cysts: aspiration temporarily reduces the bump, but the connection to the joint capsule or the mucin-producing tissue often regenerates the cyst within months.
For hormonally driven cysts like ovarian follicular cysts, recurrence depends on whether the underlying hormonal pattern persists. A single functional cyst that resolves may never return, while conditions that chronically disrupt ovulation can produce new cysts with each cycle. Genetic cysts, like those in polycystic kidney disease, recur by definition because every cell carrying the mutation has the potential to initiate a new cyst whenever its remaining functional gene copy is lost or suppressed below the critical threshold.