Skin “cysts” are most commonly epidermoid cysts (often mislabeled “sebaceous cysts”) or pilar cysts. Both are benign, walled-off sacs beneath the skin that collect keratin. They’re frequent on the scalp, face, neck, and trunk, and can occasionally become inflamed or secondarily infected if their contents leak into surrounding tissue.
- Epidermoid (“sebaceous”) cysts: These arise from the upper hair-follicle/epidermal lining, not from true sebaceous (oil) glands. A blocked follicular opening traps keratin inside, and the sac enlarges over time.
- Pilar (trichilemmal) cysts: These form from the outer root sheath of hair follicles, classically on the scalp. They’re often firm, smooth, and mobile. They can run in families.