Cysts form when something blocks a normal pathway in the body, trapping fluid, air, or tissue inside a closed sac. The blockage can come from a damaged hair follicle, a clogged gland, a joint producing too much fluid, or even a genetic mutation that disrupts how cells grow. Because cysts can develop in nearly any tissue, the specific trigger depends on where in the body the cyst appears. Most cysts are benign, meaning they contain no cancer cells and are fundamentally different from tumors.
The Basic Mechanism Behind All Cysts
At the simplest level, a cyst develops when a structure in the body, like a duct that carries or drains fluid, gets blocked. That blockage can result from infection, injury, chronic inflammation, or a problem in nearby tissue. Once the normal exit route is sealed off, fluid, blood, pus, or other material builds up and the body walls it off with a lining of tissue, creating the characteristic sac.
This is what separates a cyst from a tumor. Cysts are pouches filled with fluid or semi-solid material. Tumors are masses of abnormal cells that multiplied out of control. A cyst forms because something got stuck; a tumor forms because the cell growth cycle went wrong. Almost all cysts are benign, though in rare cases a cyst wall can contain abnormal cells, which is why doctors sometimes investigate larger or unusual-looking cysts more closely.
Skin Cysts: Keratin and Clogged Follicles
The cysts people notice most often are the ones they can see and feel under the skin. These fall into a few common types, each with a slightly different cause.
Epidermoid cysts form when skin cells that would normally shed get trapped beneath the surface. Instead of flaking away, old cells and a protein called keratin (the same protein that builds hair and nails) pile up under the top layer of skin. They keep multiplying in that enclosed space, gradually forming a slow-growing lump. A damaged or irritated hair follicle is often the starting point.
Pilar cysts work the same way but grow specifically from hair follicles, most commonly on the scalp. They’re filled with dense keratin and tend to run in families.
Sebaceous cysts originate from the oil-producing glands (sebaceous glands) found across most of the body. These glands secrete an oily substance that keeps skin moisturized. When a gland’s opening gets blocked, the oil accumulates inside, forming a small dome-shaped cyst. Squeezing one typically releases a thick, oily material.
Ovarian Cysts: When a Follicle Doesn’t Release an Egg
Functional ovarian cysts are among the most common cysts in women of reproductive age, and they’re directly tied to the menstrual cycle. Each month, a small sac called a follicle develops inside an ovary and releases an egg. A follicular cyst forms when that follicle fails to rupture and release the egg. Instead of opening, the follicle seals shut, fills with fluid, and grows into a cyst.
A second type, called a corpus luteum cyst, happens after the egg has been released. Normally, the empty follicle shrinks and dissolves. If it instead reseals and fills with fluid or blood, it becomes a cyst. Both types usually resolve on their own within a few menstrual cycles without treatment.
Joint and Tendon Cysts
Ganglion cysts are firm, round lumps that typically appear on the wrist, hand, or foot. They grow out of a joint capsule or the lining of a tendon and contain thick, jelly-like fluid similar to the lubricating fluid found inside joints. No one has identified a definitive cause. They seem to develop when joint tissue bulges outward, forming a stalk-like connection to the joint that fills with fluid.
Baker cysts (also called popliteal cysts) form behind the knee and have a clearer trigger. They’re typically caused by an underlying knee problem, like osteoarthritis, rheumatoid arthritis, or a cartilage tear. These conditions cause the knee to overproduce synovial fluid, the lubricant that helps joints move smoothly. The excess fluid migrates to a small cushioning sac behind the knee, inflating it into a visible, sometimes painful cyst. Treating the underlying joint problem is usually the path to resolving the cyst itself.
Glandular Blockages
Many cysts in the body trace back to a blocked gland. One common example is the chalazion, a small cyst that forms on the eyelid when a meibomian gland (one of the tiny oil glands along the eyelid margin) becomes obstructed. Unlike a stye, which is caused by a bacterial infection of a lash follicle, a chalazion is not infectious. The gland’s oily secretion can’t drain, leaks into surrounding tissue, and triggers a localized inflammatory reaction that produces a firm bump. Similar gland-blockage cysts can develop in salivary glands, sweat glands, and other secretory tissues throughout the body.
Trauma and Injury
Physical trauma can create cysts that wouldn’t otherwise exist. When blunt force or a crushing injury separates the skin and fat layer from the deeper tissue underneath, blood pools in the gap. If that blood collection (a hematoma) doesn’t fully reabsorb, the body walls it off with fibrous tissue, creating a chronic fluid-filled cyst. Breakdown products from red blood cells and clotting proteins inside the hematoma trigger ongoing inflammation, which causes more fluid to seep from surrounding blood vessels into the capsule, making it grow over time.
This type of injury, sometimes called a closed degloving injury, is most commonly seen after significant pelvic or thigh trauma, such as being struck by a car. It has also been documented around the knees of professional football players who absorb repetitive impacts. These post-traumatic cysts are relatively rare, but they illustrate how the body’s normal healing response can inadvertently create a persistent fluid-filled pocket.
Genetic Causes
Some people are genetically programmed to develop cysts in specific organs. Polycystic kidney disease (PKD) is the most well-known example. In the dominant form of the disease, a person inherits one faulty copy of a gene (either PKD1 or PKD2) from an affected parent. That alone isn’t enough to trigger cyst growth. A cyst forms when the remaining healthy copy of the gene in a kidney cell also becomes damaged through a random mutation over the course of a person’s life. Once both copies are lost, the cell begins to multiply abnormally, creating a fluid-filled cyst.
About 90 percent of people with autosomal dominant PKD inherited the mutation from a parent. The other 10 percent developed a new mutation with no family history of the disease. There’s also a rarer recessive form, where a child inherits a faulty gene copy from each parent (neither of whom is affected). In all forms, cysts tend to increase in both size and number throughout a person’s lifetime, gradually affecting kidney function.
Parasitic Infections
Certain parasites form cysts inside human tissue as part of their life cycle. The most significant is the pork tapeworm (Taenia solium), which causes an infection called cysticercosis. After a person accidentally swallows tapeworm eggs, typically through contaminated food or water, the larvae hatch in the intestine, enter the bloodstream, and travel to tissues like muscle, brain, and eyes. There, they form small cysts called cysticerci. When these cysts lodge in the brain or central nervous system, the condition is called neurocysticercosis, which can cause seizures and is a leading cause of epilepsy in many parts of the world.
Other parasites, including the organisms responsible for hydatid disease, use a similar strategy: embedding in organs like the liver or lungs and growing slowly over months or years inside a protective cyst wall.
Chronic Inflammation
Long-term inflammation in a joint or tissue can drive cyst formation even without a single dramatic injury. Osteoarthritis and rheumatoid arthritis both promote excess fluid production in joints, which can pool and form cysts in surrounding tissue. Chronic skin conditions like acne can lead to repeated follicle damage and cyst formation. In each case, the underlying pattern is the same: persistent irritation or damage disrupts the body’s normal drainage or containment systems, and fluid accumulates in a new, enclosed space. Addressing the source of inflammation is often the most effective way to prevent new cysts from developing.