The white stuff inside a cyst is almost always keratin, the same protein that makes up your hair and the outermost layer of your skin. It builds up inside a pocket of trapped skin cells, compacting over time into a thick, paste-like substance that looks like white toothpaste or crumbly cheese. Most people encounter this material when a cyst near the skin’s surface gets squeezed or ruptures on its own.
What Keratin Looks Like Inside a Cyst
Keratin inside a cyst doesn’t look anything like the hard, structured protein in your fingernails. Instead, it accumulates in soft, layered sheets, sometimes described in medical literature as “laminated.” When it comes out, it’s usually white to yellowish, thick, and semi-solid. The texture ranges from smooth and paste-like to chunky and crumbly, depending on how long the material has been building up and how compressed it’s become.
One detail that catches people off guard is the smell. Cyst contents often have a strong, unpleasant odor. This happens because the keratin mixes with dead skin cells, cell debris, and small amounts of cholesterol and fat as it breaks down inside the enclosed space. Bacteria on the skin can also contribute to the smell, especially if the cyst has ruptured or developed a small opening.
How the White Stuff Gets There
The two most common skin cysts, epidermoid cysts and pilar cysts, both produce keratin, but they form in slightly different ways. Epidermoid cysts develop when surface skin cells get pushed deeper into the skin, often through a hair follicle, a minor injury, or a clogged pore. Those displaced cells form a tiny sac lined with the same type of skin you have on the outside of your body. That lining keeps producing keratin the way normal skin does, but with nowhere for it to go, the material piles up inside the capsule.
Pilar cysts work the same way but originate from the outer root sheath of a hair follicle rather than from surface skin. They’re most common on the scalp. Both types produce contents that look and feel nearly identical: thick, white, cheesy keratin.
Despite being commonly called “sebaceous cysts,” most of these lumps are actually epidermoid or pilar cysts. True sebaceous cysts, which produce oily sebum rather than keratin, are rare. Sebum is a different substance entirely, made mostly of fats like triglycerides, wax esters, and a compound called squalene. The distinction matters mainly to your doctor, but it explains why you’ll sometimes see the terms used interchangeably even though the contents are chemically different.
Why Some Cysts Look Different
Not every cyst produces the same shade of white paste. Color and consistency vary based on a few factors:
- Age of the cyst. Newer cysts tend to have softer, whiter material. Older cysts can develop thicker, more yellowish or grayish contents as keratin and cholesterol deposits compact together.
- Infection or rupture. If a cyst becomes inflamed or infected, the contents may turn more yellow or greenish and become more liquid. You might also notice increased redness, swelling, warmth, and pain around the lump.
- Location. Cysts on the scalp (usually pilar cysts) often have denser, more solid keratin compared to epidermoid cysts on the face, neck, or trunk.
What Happens When a Cyst Ruptures
When a cyst wall breaks, either from pressure, squeezing, or just on its own, the keratin spills into the surrounding tissue under the skin. Your immune system treats that displaced keratin as a foreign substance and launches an inflammatory response. This is why a ruptured cyst can suddenly become red, swollen, and painful even without an actual bacterial infection present. The body is reacting to the keratin itself.
This inflammation is often mistaken for infection, but the two are different. A genuinely infected cyst involves bacteria that have entered the sac, and it typically produces more intense redness, heat, and sometimes fever. Either way, a cyst that suddenly becomes painful and swollen needs professional attention rather than home drainage, because squeezing pushes the contents deeper into the tissue and worsens the inflammatory reaction.
Why the Stuff Keeps Coming Back
Draining a cyst removes the keratin buildup, but it doesn’t remove the sac that produced it. That lining of skin cells is still intact beneath the surface, and it will continue generating keratin. This is why cysts commonly refill after being popped or drained at home. Complete removal requires excising the entire cyst wall, which is a minor surgical procedure typically done under local anesthesia. When the sac is fully removed, recurrence rates drop significantly.
A small central opening called a punctum is sometimes visible on the skin’s surface directly over a cyst. This tiny dark dot is essentially the pore or follicle where the skin cells originally became trapped. It’s one of the most reliable visual clues that a lump is a keratin-filled cyst rather than a lipoma (a fatty lump) or another type of growth.