Cysts form when fluid, protein, or debris gets trapped inside a closed sac of tissue beneath the skin or within an organ. The specific trigger depends on where the cyst develops, but the underlying pattern is consistent: something that should flow freely or shed naturally gets sealed off instead, and a pocket builds up around it. Most cysts are benign, and many people develop at least one during their lifetime without ever knowing it.
How Skin Cysts Form
The most common cysts people notice are the firm, round bumps that appear just under the skin. These are usually epidermal cysts (sometimes called sebaceous cysts, though that name is technically inaccurate since they’re filled with keratin, not oil). Your skin constantly sheds dead cells from its outer layer. Normally those cells work their way to the surface and flake off. When something disrupts that process, the cells can get pushed deeper instead of outward, becoming trapped beneath the surface.
Once trapped, these cells form the walls of a small sac and begin secreting keratin, a thick protein your body uses to build skin, hair, and nails. The keratin accumulates inside the sac, creating that characteristic dense, cheese-like substance. Over time the cyst grows as more material collects.
Several things can set this process in motion:
- Blocked hair follicles. A blockage at the top of a hair follicle is the most common trigger. Acne, chronic sun damage, and other skin conditions can all cause these blockages.
- Skin injuries. A scratch, surgical wound, or any trauma that pushes surface cells into the deeper layer of skin (the dermis) can seed a cyst. This is why cysts sometimes appear at the site of an old scar or piercing.
- Areas without hair follicles. Even on skin with no hair, an injury can force surface cells below the top layer, where they become trapped and start accumulating keratin.
Ovarian Cysts and Hormonal Triggers
Ovarian cysts are extremely common and usually develop as a normal part of the menstrual cycle. Each month, an ovary grows a small follicle that holds a developing egg. When ovulation works properly, the follicle ruptures, releases the egg, and dissolves. When it doesn’t, a cyst can form in one of two ways.
A follicular cyst develops when the follicle never ruptures or releases its egg. Instead of breaking open, it keeps growing and fills with fluid. These are the most common type and typically resolve on their own within a few menstrual cycles.
A corpus luteum cyst forms after the egg has been released. Normally the empty follicle shrinks and produces hormones to support a potential pregnancy. But sometimes the opening where the egg escaped seals shut, trapping fluid inside. The follicle swells into a cyst that can grow several centimeters across.
Hormonal factors raise the risk. Fertility medications that stimulate ovulation can increase the chance of cyst formation because they push the ovaries to develop more follicles than usual. Pregnancy can also play a role: the follicle that forms during ovulation sometimes persists on the ovary throughout the pregnancy rather than dissolving as it normally would.
Ganglion Cysts on Joints and Tendons
Ganglion cysts are the rubbery lumps that pop up on wrists, hands, and sometimes feet. They grow out of the tissues surrounding a joint or tendon sheath, almost like a balloon inflating on a stalk. Inside that balloon is a thick, slippery fluid similar to the lubricant your joints naturally produce.
What triggers their formation isn’t fully understood, but repetitive stress and joint irritation seem to play a role. They tend to grow larger with increased wrist or hand activity and shrink with rest. Women develop ganglion cysts more often than men, and they’re most common between ages 20 and 40. Many ganglion cysts are painless and disappear without treatment, though they can press on nearby nerves and cause discomfort.
Pilonidal Cysts and Ingrown Hairs
Pilonidal cysts develop near the tailbone, in the crease between the buttocks. The cause is surprisingly mechanical: loose hairs puncture the skin and burrow inward. Friction from tight clothing, prolonged sitting, or activities like cycling pushes the hair deeper. Your body recognizes the embedded hair as a foreign object and builds a cyst around it in an attempt to push it out.
This is why pilonidal cysts are far more common in people with thick or coarse body hair, and in those who sit for long stretches (truck drivers and office workers, for instance). They typically affect younger adults and are more frequent in men. Once a pilonidal cyst becomes infected, it fills with pus and can be quite painful, often requiring drainage.
Cysts Caused by Infections and Parasites
Some cysts are the body’s response to an invader. Certain parasitic infections cause cysts to form inside organs, most notably a condition called cystic echinococcosis (hydatid disease). This happens when a person accidentally swallows the eggs of a tiny tapeworm, usually through contaminated food, water, or soil, or through direct contact with infected animals like dogs or livestock.
The eggs hatch inside the body and the larvae migrate to organs, most often the liver and lungs, where they slowly develop into fluid-filled cysts. These hydatid cysts can grow for years without causing symptoms, sometimes reaching 10 centimeters or more before they’re discovered. The related form, alveolar echinococcosis, has an asymptomatic incubation period of 5 to 15 years and produces a slow-growing mass usually in the liver. While these parasitic cysts are rare in many developed countries, they remain a significant health concern in pastoral communities worldwide.
Genetic Causes of Cyst Formation
Some people are genetically predisposed to develop cysts. The clearest example is polycystic kidney disease (PKD), one of the most common inherited conditions. Mutations in specific genes disrupt the chemical signaling that guides normal kidney development. Without those signals functioning properly, thousands of fluid-filled cysts form throughout both kidneys over time, gradually replacing healthy tissue and impairing kidney function.
The dominant form of PKD, which typically shows symptoms in adulthood, is caused by mutations in one of two genes. The recessive form, caused by a different gene mutation, is rarer and more severe, often appearing in infancy or childhood. Beyond PKD, mutations in structural proteins like collagen and laminin can also contribute to excessive cyst formation in other organs. Cyst development is actually a normal part of how kidneys, the liver, and the pancreas form during embryonic development. Problems arise when that process happens uncontrollably due to genetic errors.
How Cysts Are Diagnosed
Most skin cysts are diagnosed by physical examination alone. For internal cysts or lumps that need further evaluation, ultrasound is typically the first imaging tool used. A cyst that appears uniform and purely fluid-filled on ultrasound or CT scan is almost always benign and generally just needs monitoring. If a cyst contains solid components mixed with the fluid, further evaluation is needed to determine whether it’s benign or potentially something more serious.
This distinction matters because cysts and tumors can sometimes look similar from the outside. A tumor is a solid mass of tissue, while a cyst is a fluid-filled sac. The two are fundamentally different structures, and imaging can usually tell them apart quickly.
Treatment: Drainage vs. Removal
Not every cyst needs treatment. Many resolve on their own, particularly ovarian cysts and small ganglion cysts. When a cyst does need medical attention, it’s usually because it hurts, is infected, interferes with movement, or is cosmetically bothersome.
There are two main approaches. Drainage involves puncturing the cyst and removing the fluid or material inside. This provides fast relief and can help clear an infection, but it’s often temporary. If the sac lining remains intact beneath the skin, the cyst can refill over weeks or months. Surgical excision removes the entire cyst, including its outer lining. By taking out the sac completely, this approach significantly lowers the chance of recurrence. For skin cysts that keep coming back after drainage, full excision is usually the more permanent solution.