Most cysts under the skin form when surface skin cells get pushed inward instead of shedding off normally. Your body constantly produces new skin cells and discards old ones, but when that process goes wrong, cells can end up trapped beneath the surface. Once there, they keep multiplying, build a tiny walled-off pocket, and fill it with a thick protein called keratin. The result is a slow-growing, firm lump you can usually feel just under the skin.
How Skin Cells Get Trapped
Your outermost layer of skin, the epidermis, is a thin sheet of cells that replaces itself continuously. Old cells are supposed to flake away. In most cysts, something disrupts that cycle. Cells migrate deeper into the skin instead of shedding from the surface, or they get buried when the skin above them closes over. Once embedded, these cells behave exactly as they would on the surface: they multiply, form layers, and produce keratin, the same tough protein that makes up your hair and nails.
Over time, the displaced cells organize into a self-contained sac. The cells form the wall of the cyst, and the keratin they secrete fills the interior. That keratin is the thick, yellowish, cheese-like substance people sometimes see if a cyst ruptures or is squeezed. The cyst has no outlet, so the material just accumulates, and the lump slowly grows.
Common Triggers
Anything that damages or irritates the skin can set this process in motion. A cut, scrape, surgical wound, or even chronic friction can push surface cells into deeper layers. Clogged or inflamed hair follicles are another frequent starting point. The opening of the follicle gets blocked or damaged, trapping the cells that line it. Acne-related inflammation is a common culprit, particularly in areas prone to breakouts like the face, neck, and upper back.
Some cysts appear with no obvious trigger at all. A small number are thought to form before birth, when clusters of skin cells get misplaced during embryonic development. These may not become noticeable until years later.
Different Cyst Types, Different Origins
Not all lumps under the skin start the same way. The most common type, the epidermoid cyst, originates from skin cells and is lined with tissue identical to the upper portion of a hair follicle. These are often incorrectly called “sebaceous cysts,” but true sebaceous cysts are actually a separate and much less common entity. A true sebaceous cyst starts in the oil gland itself and fills with sebum, the oily substance that normally lubricates skin and hair, rather than keratin.
Pilar cysts are another variety that grows directly from hair follicles. They show up most often on the scalp, though they can appear on the face, neck, arms, or legs. Like epidermoid cysts, pilar cysts form when old skin cells and keratin pile up beneath the surface instead of being shed. The difference is mainly in the specific layer of the follicle they originate from and the texture of their contents.
Genetics and Recurring Cysts
A single cyst is usually a one-off event, but some people develop many cysts over time. In certain cases, this runs in families. A condition called steatocystoma multiplex causes numerous small cysts to grow from the oil glands across the body. It’s caused by a mutation in the gene responsible for producing keratin 17, a structural protein that helps skin cells form stable networks. When that protein is defective, cells in the oil glands grow abnormally and form sebum-filled cysts.
Only one copy of the altered gene is needed to cause the condition, meaning a parent with the mutation has a 50 percent chance of passing it on. A related disorder called pachyonychia congenita involves the same gene and also produces cysts, along with thickened nails and other skin changes. For most people with an occasional cyst, though, genetics play little or no role.
What a Cyst Feels and Looks Like
A typical skin cyst is a round, firm bump that sits just under the surface and moves slightly when you press on it. It grows slowly, sometimes over months or years, and is usually painless. Most range from pea-sized to a few centimeters across, though they can occasionally grow larger. You might notice a tiny dark dot at the center, which is the blocked pore or follicle opening where the cyst originated.
Problems arise if a cyst ruptures beneath the skin. When the keratin inside leaks into surrounding tissue, the body treats it as a foreign substance and mounts an inflammatory response. The area becomes red, swollen, warm, and tender. This is not always an infection, but the inflammation can look identical to one, and a true bacterial infection can develop on top of it. A cyst that was previously painless and suddenly becomes sore and swollen has likely ruptured internally or become infected.
Why Draining Alone Doesn’t Work
Squeezing or draining a cyst provides only temporary relief. The sac wall is still intact beneath the skin, and as long as it remains, it will continue producing keratin and refill. This is why cysts so often “come back” after people try to pop them at home.
Surgical removal of the entire cyst wall is the standard approach for a permanent fix. Two common techniques, a small punch incision and a traditional elliptical cut, both produce low recurrence rates of roughly 3 percent or less when the full sac is removed. The procedure is typically minor, done under local anesthesia in an office setting, and recovery is straightforward.
If a cyst is painless, not growing, and not in a cosmetically bothersome spot, leaving it alone is a perfectly reasonable option. Many people live with small cysts for years without any issues. Removal becomes more practical when a cyst keeps getting inflamed, sits in a spot where it’s frequently irritated by clothing or movement, or is large enough to be a nuisance.