Noticing a new lump or bump on your skin is never a happy discovery, especially if it’s something unfamiliar — not your garden-variety wart, mole, or hives — and you’re not sure whether to worry.

Even the term “lump” or “bump” can be confusing. Located on or just under the skin, these growths can be almost flat or “cause the skin to pooch out over them,” says Dr. Rachel Reynolds, interim chair of dermatology at Harvard-affiliated Beth Israel Deaconess Medical Center. Beyond that, their characteristics run the gamut: soft and squishy or rock-hard, movable or fixed in place, round or irregularly shaped, or growing slowly or quickly.

Regardless, they seldom signal something serious. “People feel a lump and often immediately think it’s cancer,” Dr. Reynolds says. “But that’s quite rare.”

Most common types

Aside from highly common basal and squamous cell skin cancers — which can appear as shiny or wartlike domes — the vast majority of other skin bumps are either epidermal cysts or lipomas, Dr. Reynolds says. What are their qualities?

Epidermal cysts often appear on the face or back and “may feel like a marble under the skin,” she says. They grow as benign “buds” off hair follicles, filling with a cheesy protein called keratin. (Pilar cysts are another version, but on the scalp.) As they grow, epidermal cysts can become inflamed or infected.

“Sometimes, even after being present for many years, they act like a boil, enlarging and becoming red and tender,” Dr. Reynolds says. “If that happens, see a doctor, since some will require drainage or antibiotics.”

Lipomas are benign fatty tumors that often appear on the trunk or shoulders. They usually grow slowly and can run in families. “You can’t see most lipomas unless they get really big,” she says. “But if you are able to feel them, they’re really squishy, without any sharp edges.”

A lipoma can become uncomfortable if it presses on surrounding nerves. In this case, your doctor can remove it surgically or with liposuction, or dissolve it by injecting it with a drug called deoxycholic acid (Kybella).

Diverse list

Other types of skin growths are defined by a variety of diverse features.

Cherry angiomas are smooth, red bumps representing an overgrowth of superficial blood vessels in the skin. They can range in size from a tiny dot to the diameter of a pencil eraser. Cherry angiomas tend to appear suddenly, especially on the torso, and are more common in people over 40.

Dermatofibromas are small, firm, brown or pink spots that contain scar tissue, sometimes appearing after a skin injury or insect bite. They might itch or pucker inward if you pinch them, but they aren’t painful.

Keloids occur when scar tissue continues to grow beyond an injured area. These large bumps aren’t harmful, although “they can become itchy and unsightly,” she says.

Sarcomas are cancerous tumors that can appear deceptively similar to cysts or lipomas, but typically grow more quickly. “A sarcoma may feel firmer under the skin, but not like a marble, as an epidermal cyst does,” Dr. Reynolds says. “It’s a little more ill-defined, and sometimes it’s painful.” If the lump hurts even when it’s left alone, that’s a particularly concerning sign.

When to act

While sarcomas are one of the few types of skin lumps that are dangerous, Dr. Reynolds urges caution about any lump appearing near lymph nodes — immune system nodules located in the armpits, groin, and neck. While lymph nodes can enlarge from a cold or other infection, swollen nodes sometimes signal cancer. Other symptoms might also be present, including weight loss, fever, and chills.

“Get checked,” she says. “Some lymph nodes become temporarily inflamed, enlarge, and then go back down, but it’s something that should be monitored.”

A dermatologist can usually diagnose a skin bump or lump on sight alone. Occasionally, imaging tests or a biopsy may be needed to determine the culprit. You can be proactive by doing the following:

Pointing it out. At your next primary care appointment, ask your doctor to take a look. “Seek a more urgent visit if something is growing quickly, bleeding, feels painful, looks infected, or is red or tender,” Dr. Reynolds says.

Speaking up. Your doctor might not focus on a benign bump unless you mention that it’s painful, irritating, or cosmetically unappealing.

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