Most bumps under the skin are harmless, slow-growing masses like cysts or lipomas. They’re extremely common, and the type you’re dealing with usually depends on where the bump is, how it feels when you press on it, and how quickly it appeared. Understanding these details can help you figure out what’s going on and whether it needs attention.
Lipomas: Soft, Moveable Lumps
Lipomas are one of the most common causes of a bump under the skin. They’re benign growths made of fat cells that sit a bit deeper than other types of bumps. When you press on a lipoma, it feels soft and slightly squishy, and you can usually nudge it around under the skin with your fingers. They grow slowly, sometimes over years, and can appear almost anywhere on the body, though the neck, shoulders, back, and arms are typical spots.
Lipomas rarely cause pain unless they press on a nearby nerve. Most people notice them only because they can feel an unfamiliar lump. They don’t need treatment unless they’re bothersome or growing, in which case a doctor can remove them with a simple procedure.
Cysts: Firm, Round, and Fixed in Place
Epidermoid cysts (sometimes called sebaceous cysts) are another frequent culprit. These form when skin cells get trapped beneath the surface and create a sac filled with a thick, protein-rich material. Unlike lipomas, cysts feel firmer, more like an egg or a piece of rubber under the skin. They tend to stay put when you touch them rather than sliding around.
Cysts form slowly and can sit unchanged for months or years. They’re painless unless they become inflamed or infected, at which point they may turn red, warm, and tender. A common frustration with cysts is that simply draining them often isn’t enough. If the sac wall is left behind, the cyst tends to refill. Complete surgical removal of the cyst wall, by contrast, has a recurrence rate of only about 3%.
Ganglion Cysts Near Joints
If your bump is on or near a wrist, hand, ankle, or foot, it may be a ganglion cyst. These round or oval lumps form along tendons or joints and are filled with a thick, jelly-like fluid. They can be as small as a pea or grow larger, and their size sometimes fluctuates, getting bigger with repetitive motion and shrinking with rest.
Ganglion cysts can be painless or cause a dull ache if they press on a nerve. They sometimes resolve on their own. If one is causing discomfort or limiting your range of motion, a doctor can drain it with a needle or remove it surgically.
Swollen Lymph Nodes
A bump that appears quickly in your neck, armpit, or groin is more likely a swollen lymph node than a cyst. Lymph nodes are small immune system glands clustered in predictable zones: the neck, underarms, around the collarbone, and the groin. When your body is fighting an infection, nearby nodes swell, sometimes noticeably.
Swollen lymph nodes feel softer and more rubbery than cysts. They shift slightly when pressed and are often tender. The key difference is timing. Lymph nodes swell quickly in response to illness and typically shrink back to normal within days or weeks as you recover. Cysts, on the other hand, develop gradually and don’t go away on their own. If a lymph node stays swollen for more than two to three weeks without an obvious cause like a cold, it’s worth getting checked.
Keratin Plugs and “Chicken Skin”
Not all under-skin bumps are individual lumps. If you have clusters of tiny, rough bumps on your upper arms, thighs, or buttocks, you’re likely dealing with keratin plugs, the hallmark of a condition called keratosis pilaris. These form when dead skin cells and a protein called keratin clog hair follicles, creating small raised bumps that give the skin a sandpaper-like texture. Some people describe it as “chicken skin.”
Keratin plugs are more common when skin is dry or irritated by friction. They’re completely harmless and tend to improve with regular moisturizing and gentle exfoliation. Many people notice them more in winter when humidity drops.
Folliculitis and Ingrown Hairs
Small, painful bumps that appear in areas where you shave or where skin rubs together may be infected hair follicles (folliculitis) or ingrown hairs. Bacterial folliculitis shows up as itchy, pus-filled bumps when hair follicles get infected, usually by staph bacteria. It can occur anywhere you have hair but is especially common on the thighs, buttocks, and scalp.
Ingrown hairs, sometimes called razor bumps, look similar but have a different cause. Instead of infection, a hair curls back into the skin as it grows, creating an inflamed bump. This is most common in people with curly hair and typically appears on the face and neck after shaving. Letting the hair grow out, using a single-blade razor, and avoiding shaving too close can prevent recurrence.
Dermatofibromas
A small, firm bump on your lower leg that dimples inward when you pinch it is likely a dermatofibroma. These benign growths develop slowly in the skin and are most common on the extremities, particularly the legs. They’re usually brownish or reddish, less than a centimeter across, and completely painless.
The characteristic “dimple sign” happens because the bump is tethered to the skin above it. When you squeeze the skin on either side, the surface dips inward rather than popping outward. Dermatofibromas don’t require treatment and are harmless, though they rarely go away on their own. A doctor can remove one if it’s in an annoying spot or if you want it gone for cosmetic reasons.
Less Common Causes
Neurofibromas are benign tumors that grow from nerve tissue, typically appearing in the skin or just beneath it. A single neurofibroma is usually nothing to worry about. Multiple neurofibromas, however, can be a sign of a genetic condition called neurofibromatosis, which requires ongoing monitoring because these growths can, in rare cases, become cancerous over time.
When a Bump Needs Medical Attention
The vast majority of under-skin bumps are benign, but certain features warrant a closer look. A bump that is larger than a golf ball, growing steadily, or feels hard and immovable is worth having evaluated. A lump that was previously removed and grows back in the same spot also deserves attention, as this can occasionally signal a soft tissue sarcoma.
Imaging tests like ultrasound, CT, or MRI can help characterize a mass, but they can’t definitively distinguish a cancerous growth from a benign one. For that, a biopsy (removing a small tissue sample for examination under a microscope) is the standard. Your doctor may use a needle biopsy for lumps that can be felt through the skin, or an image-guided approach for deeper masses. A bump that’s been stable for years, feels soft, and moves freely under the skin is almost certainly benign, but if anything changes, size, firmness, or sensation, that’s the time to get it evaluated.