Cysts form when cells, fluid, or other substances get trapped beneath the surface of your skin or inside an organ with no way to drain. They’re essentially closed sacs that your body builds around something it can’t easily expel. The specific trigger depends on where the cyst appears, but the underlying pattern is almost always the same: something that should have moved through your body gets stuck, and tissue walls off around it.
What’s Actually Inside a Cyst
A cyst is a closed, usually round sac that can contain fluid, air, oil, or a thick protein called keratin. Despite what many people assume, most skin cysts aren’t filled with pus or fat. The most common type, epidermoid cysts, are packed with keratin, the same tough protein that makes up your hair and nails. It has a thick, cheese-like consistency, which is why squeezing a cyst produces that distinctive pale, dense material rather than clear fluid.
Your skin constantly sheds old cells from its surface. Sometimes those cells migrate inward instead of outward, embedding themselves deeper in the skin. Once trapped, they continue doing what skin cells do: producing keratin. That keratin accumulates with nowhere to go, and the surrounding tissue forms a sac around it. The result is a firm, slow-growing lump just beneath the skin.
Not all cysts contain keratin, though. Ganglion cysts, which develop near joints and tendons, are filled with a thick jelly-like fluid similar to the lubricant in your joints. Ovarian cysts contain the fluid that normally surrounds a developing egg. Sweat gland cysts hold residual sweat. The contents depend entirely on which type of cell or gland produced the cyst.
Skin Injuries and Blocked Follicles
The most common reason skin cysts develop is some form of disruption to the skin’s surface. A scratch, surgical wound, acne, or even chronic sun damage can interrupt the normal path that dead skin cells take as they shed. Instead of sloughing off, cells get pushed below the top layer of skin into the deeper layer (the dermis), where they form a pocket and start accumulating keratin.
Hair follicles are another frequent starting point. When a follicle gets irritated or damaged, the cells lining it can turn inward and create a small enclosed space. This is why epidermoid cysts often appear on the face, neck, and trunk, areas dense with hair follicles. Epidermoid cysts account for roughly 80 to 90 percent of all skin cysts and are the fifth most common skin diagnosis across all medical specialties in the United States. They can show up at any age but most commonly appear between ages 20 and 50.
On parts of your body without hair follicles, cysts still form after injury or trauma pushes surface cells into deeper tissue. The mechanism is the same: displaced cells, trapped keratin, a sac that slowly grows.
Hormonal Triggers and Ovarian Cysts
Inside the body, hormones are the most common cyst trigger. Ovarian cysts are a prime example. Each month, an egg develops inside a small fluid-filled structure called a follicle. Normally the follicle bursts to release the egg, then shrinks and dissolves. But sometimes the follicle doesn’t release the egg, or it doesn’t discharge its fluid afterward. When that happens, the follicle swells and becomes a cyst.
These “functional” ovarian cysts are tied directly to the menstrual cycle and are extremely common in anyone who hasn’t gone through menopause. Most resolve on their own within a few weeks without causing symptoms. Polycystic ovary syndrome (PCOS) is a related but distinct condition where altered hormone levels prevent multiple follicles from maturing to the point of releasing an egg. Those undeveloped follicles remain as many small cysts on the ovaries.
Hormonal shifts during puberty, menstruation, pregnancy, and menopause can all influence cyst development, both on the ovaries and on the skin. Stress-related hormone changes play a role too, particularly in cystic acne, where deep, inflamed cysts form in the skin’s oil glands.
Genetics and Chronic Cyst Conditions
Some people develop cysts repeatedly because of their genetic makeup. Steatocystoma multiplex is a condition caused by mutations in a gene called KRT17, which provides instructions for building a specific type of keratin. People with this mutation develop dozens or even hundreds of small cysts filled with sebum, the oily substance that normally lubricates skin and hair. These cysts originate in the oil glands themselves and typically first appear during adolescence, most often on the torso, neck, upper arms, and upper legs.
The condition is inherited in an autosomal dominant pattern, meaning only one copy of the altered gene (from one parent) is enough to cause it. Researchers consider steatocystoma multiplex a variant of a broader condition called pachyonychia congenita, which involves both cyst growth and nail abnormalities. These are the cases where the term “sebaceous cyst” actually applies, since the cysts are truly filled with sebum rather than keratin.
Joint Stress and Ganglion Cysts
Ganglion cysts are the most common type of lump found on the hand and wrist, though they can also appear near other joints and tendons. They grow out of the lining of a joint or tendon and look like a small, firm balloon under the skin. Inside is a thick, gel-like fluid similar to the synovial fluid that cushions your joints.
The exact cause isn’t fully understood, but prior injury to a joint or tendon significantly increases the risk. Repetitive stress to the wrist or hand is a common pattern among people who develop them. Unlike skin cysts, ganglion cysts can fluctuate in size, sometimes swelling with activity and shrinking with rest. They’re not cancerous and often disappear on their own, though ones that press on a nerve can cause pain or tingling.
Cysts vs. Abscesses
A lump under the skin isn’t always a cyst. Abscesses look similar but have a completely different cause. A cyst is a closed sac filled with non-infectious material like keratin or fluid. It’s typically firm to the touch and grows slowly. An abscess is a pocket of pus caused by a bacterial infection. White blood cells rush to the infected area, and the resulting mixture of living and dead cells, bacteria, and damaged tissue collects into a soft, painful mass.
Skin abscesses usually start from bacteria entering through a cut, scrape, or ingrown hair. They tend to be warm, red, and tender, and they develop faster than cysts. A cyst can become infected and turn into something resembling an abscess, but on its own, a cyst is sterile and painless unless it’s pressing on surrounding tissue.
Risk Factors That Increase Your Chances
Several factors make cyst development more likely. A history of acne is one of the strongest predictors for skin cysts, since acne creates exactly the kind of follicle damage and skin disruption that traps cells beneath the surface. Family history matters too, both for genetic conditions like steatocystoma multiplex and for cystic acne, which runs strongly in families.
Age plays a role depending on the type. Epidermoid cysts peak during adulthood, while tiny epidermoid cysts called milia are more common in newborns. Cystic acne hits hardest during the teenage years when hormonal surges are at their peak. Hormonal changes during menopause and periods of high stress can also trigger flares. Diets high in simple sugars and dairy have been linked to increased acne and cyst-related breakouts, as have oily work environments like commercial kitchens.
What Happens When a Cyst Is Treated
Many cysts never need treatment. Small, painless cysts that aren’t growing or causing cosmetic concerns can simply be left alone. The problem is that draining a cyst, whether at home or in a medical setting, rarely solves the issue permanently. If the sac wall remains intact beneath the skin, the cyst will almost always refill.
For cysts that are inflamed or infected, the standard approach is incision and drainage. This relieves pressure and discomfort quickly. Antibiotics are added only when there are signs of spreading infection, like fever or significant redness extending beyond the cyst. For cysts that keep coming back, complete surgical removal of the sac wall is the most reliable way to prevent recurrence. Without removing that lining, the cells that produced the cyst in the first place continue doing their job.
Picking at, squeezing, or popping a cyst at home is one of the most common ways people turn a harmless lump into an infected one. The pressure can push contents deeper into surrounding tissue, trigger inflammation, and introduce bacteria through broken skin. If a cyst becomes red, warm, rapidly growing, or painful, those are signs it needs professional attention rather than home management.