Cysts form when fluid, air, or solid material like keratin (a protein your skin naturally produces) gets trapped inside a closed sac of tissue. The specific cause depends on where the cyst develops, but the most common triggers are blocked ducts, physical injury, chronic inflammation, infections, hormonal shifts, and genetic mutations. Some cysts are completely predictable parts of aging, while others seem to appear out of nowhere.
How Cysts Form at a Basic Level
A cyst is a round, enclosed pocket surrounded by its own wall of cells. Unlike a tumor, which is a solid mass of growing tissue, a cyst has a distinct sac around it and is almost always noncancerous. The contents vary: skin cysts are typically filled with keratin or an oily substance called sebum, joint-related cysts contain thick, jelly-like fluid, and organ cysts are often filled with clear fluid.
The underlying process usually involves one of a few pathways. A duct or pore gets plugged and material backs up behind it. Tissue gets damaged and surface cells end up trapped beneath the skin. A joint produces too much fluid and it pools in a weak spot. Or a hormonal signal misfires and a normal structure, like an egg follicle, fails to open and release its contents. Each of these pathways leads to a different type of cyst, but they all share the same basic architecture: a walled-off pocket that slowly fills.
Skin Cysts: Blocked Pores and Trapped Keratin
The most common cysts people notice are the firm, round lumps that develop just under the skin. These are usually epidermoid cysts, and they originate from the upper part of a hair follicle. When the follicle’s opening gets plugged, the skin cells lining it keep producing keratin with nowhere for it to go. The keratin accumulates in the deeper layers of skin, forming a slow-growing lump that can range from pea-sized to over an inch across.
Anything that disrupts a hair follicle can set this process in motion. Acne is a major contributor: people with acne vulgaris often develop multiple epidermoid cysts that grow out of clogged pores called comedones. Physical trauma matters too. A cut, a scrape, or even a surgical incision can push surface skin cells into deeper tissue where they don’t belong. Once implanted there, those cells continue producing keratin, creating what’s called an epidermal inclusion cyst. This is why cysts sometimes appear at the site of an old injury or along a surgical scar.
If a skin cyst ruptures beneath the surface, the soft keratin inside spills into surrounding tissue and triggers an inflammatory reaction. The area becomes red, swollen, and painful, which is often the point when people first seek treatment. The cyst itself wasn’t infected, but the body treats the displaced keratin as a foreign material.
Pilar Cysts
Pilar cysts are a close relative of epidermoid cysts, but they originate from a different part of the hair follicle, specifically between the oil gland and the tiny muscle that makes hair stand up. They occur in less than 10 percent of the population and show up almost exclusively on the scalp. They tend to run in families and often appear in clusters.
Ganglion Cysts: Joint Stress and Fluid Leaks
Ganglion cysts are the rubbery lumps that commonly develop on the wrist, hand, or foot near a joint or tendon. The leading theory is that stress on a joint, whether from a single injury or repetitive use, creates a small tear in the joint capsule. Synovial fluid (the lubricant inside your joints) leaks through that tear into the surrounding tissue. The leaked fluid reacts with local tissue, thickens into a gel-like substance, and gradually forms a cyst wall around itself.
Pre-existing joint problems make this more likely. Ligament injuries around the wrist, for example, can alter how forces travel through the joint, eventually weakening the capsule at a specific point and allowing fluid to escape. A second theory suggests that joint stress causes the connective tissue near the joint to break down on its own, producing mucus-like fluid that pools and forms a cyst without any actual tear in the capsule. Either way, repetitive mechanical strain is the common thread.
Ovarian Cysts: Hormonal Cycles Gone Slightly Off
Most ovarian cysts are functional, meaning they’re a byproduct of the normal menstrual cycle rather than a disease. Each month, an egg develops inside a small fluid-filled structure called a follicle. Normally, the follicle bursts to release the egg and then shrinks. A cyst forms when this process stalls: either the follicle doesn’t release the egg, or it releases the egg but fails to drain its fluid and collapse afterward. The follicle swells and becomes a cyst, typically resolving on its own within a few cycles.
In polycystic ovary syndrome (PCOS), altered hormone levels prevent follicles from maturing enough to ovulate. The result is many small cysts on the ovaries, each one an egg follicle that started developing but never reached the point of release. PCOS affects hormone balance broadly, which is why it’s associated with irregular periods, excess androgen levels, and metabolic changes beyond the cysts themselves.
Baker’s Cysts: Excess Knee Fluid
A Baker’s cyst (also called a popliteal cyst) is a fluid-filled bulge behind the knee. It’s almost always a secondary problem, meaning something else inside the knee is driving it. Osteoarthritis, rheumatoid arthritis, meniscus tears, and ACL tears all cause the knee joint to produce excess synovial fluid as part of the inflammatory response. That extra fluid travels to the back of the knee and exits through a one-way valve into the space behind the joint, where it pools and forms a visible, sometimes uncomfortable swelling.
Because the valve only allows fluid to flow out, not back in, the cyst tends to persist or grow as long as the underlying joint problem keeps generating excess fluid. Treating the Baker’s cyst alone without addressing the arthritis or torn cartilage usually means it comes back.
Kidney Cysts: A Side Effect of Aging
Simple kidney cysts are among the most common cysts in the body, and they become dramatically more prevalent with age. About 25 percent of people over 40 have at least one, and that number climbs to roughly 50 percent by age 50. These cysts are acquired, not inherited, and they grow slowly, potentially doubling in size over a decade.
The exact mechanism isn’t fully understood, but the prevailing theory involves tiny outpouchings on the kidney’s filtering tubes. As you age, the structural membrane supporting these tubes gradually weakens. Combine that with minor degrees of urinary obstruction, and fluid begins to accumulate in these small outpouchings, slowly expanding them into cysts. The vast majority are harmless and discovered incidentally on imaging done for something else entirely.
Genetic Causes of Recurring Cysts
When cysts keep coming back or appear in large numbers, genetics may be involved. One well-documented example is steatocystoma multiplex, a condition caused by mutations in the KRT17 gene, which provides instructions for building a structural protein called keratin 17. The mutation prevents keratin 17 from forming stable networks within skin cells, disrupting the normal function of oil glands. The result is dozens or even hundreds of small, sebum-filled cysts, typically appearing on the chest, arms, and neck in adolescence or early adulthood.
This condition follows an autosomal dominant inheritance pattern, meaning you only need one copy of the altered gene to develop it. If one parent carries the mutation, each child has a 50 percent chance of inheriting it. Steatocystoma multiplex is considered by many researchers to be a variant of a broader condition called pachyonychia congenita, which involves the same gene and also affects the nails.
Other genetic pathways also contribute to cyst formation at a cellular level. Signaling systems that control how hair follicles develop and maintain themselves can malfunction due to specific gene deletions, causing follicles to degenerate into cysts instead of producing hair. These mechanisms help explain why some people are simply more prone to cysts than others, even without a named genetic syndrome.
How Cysts Differ From Abscesses
Cysts and abscesses can look similar from the outside, but they have different causes and behave differently. A cyst is a firm, slow-growing lump filled with fluid or keratin. It’s not caused by infection and isn’t contagious. An abscess is a pocket of pus caused by bacterial infection. It’s typically red, warm, tender, soft to the touch, and may drain on its own if the skin breaks. The pus from an abscess is highly contagious.
The distinction matters because treatment is different. A cyst that becomes inflamed can mimic an abscess, but true abscesses require addressing the bacterial infection, while cysts that aren’t bothering you can often be left alone. If a lump is growing rapidly, feels warm, or is increasingly painful, those features point more toward infection than a simple cyst.