A skin lesion is any area of skin that looks or feels different from the surrounding tissue. That includes everything from a freckle to a blister to a scab. The term is intentionally broad, covering spots, bumps, sores, and patches of all kinds. Most skin lesions are harmless, but some signal infection, autoimmune disease, or skin cancer, which is why understanding the basics helps you know what deserves attention.
Primary vs. Secondary Lesions
Dermatologists divide skin lesions into two broad categories. Primary lesions are the original change in your skin, the thing that appeared on its own. Secondary lesions develop when a primary lesion is altered by scratching, infection, healing, or an underlying condition getting worse. A blister is a primary lesion; the crust that forms after it pops is a secondary one.
This distinction matters because it helps clinicians trace what happened to your skin and why. A scaly patch, for instance, tells a different story than the smooth red spot it may have started as.
Types of Primary Lesions
Primary lesions are classified by their size, shape, and whether they sit flat against the skin or rise above it. Here are the main types:
- Macule: A flat, discolored spot smaller than 10 mm. Freckles and flat moles are macules. A larger version (bigger than 10 mm) is called a patch.
- Papule: A small raised bump under 10 mm that you can feel with your fingertip. Warts and some types of acne are papules.
- Plaque: A raised or depressed area larger than 10 mm. Psoriasis patches are a classic example.
- Nodule: A firm bump that extends deeper into the skin than a papule, reaching into lower tissue layers.
- Vesicle: A small fluid-filled blister under 10 mm. Cold sores and chickenpox spots start as vesicles.
- Bulla: A fluid-filled blister larger than 10 mm, like a severe burn blister.
- Pustule: A blister filled with pus rather than clear fluid. Acne whiteheads are pustules.
- Wheal: A raised, swollen area caused by fluid collecting just beneath the skin surface. Hives are wheals.
The 10 mm threshold (roughly the width of a pencil eraser) is the key dividing line between several of these categories. Knowing whether a spot is flat or raised, solid or fluid-filled, and small or large narrows down the possible causes considerably.
Types of Secondary Lesions
Secondary lesions are what happen after. They develop when a primary lesion is scratched, rubbed, infected, or simply goes through its natural healing process. Common secondary lesions include:
- Crust: Dried fluid (blood, serum, or pus) that hardens over a wound. Scabs are crusts.
- Scale: Flaky, peeling skin, often seen with eczema or psoriasis.
- Erosion: A shallow loss of the top skin layer, leaving a deflated, slightly raw surface.
- Ulcer: A deeper open sore that extends below the surface and is often slow to heal.
- Fissure: A narrow, painful crack in the skin, common on heels or around the corners of the mouth.
What Causes Skin Lesions
The list of causes is enormous because “skin lesion” covers so much ground. Broadly, they fall into a few categories. Infections from bacteria, viruses, or fungi cause lesions like boils, warts, cold sores, and ringworm. Allergic reactions produce hives, contact dermatitis rashes, or drug reaction spots. Autoimmune conditions, where the immune system mistakenly attacks the body’s own cells, cause lesions in diseases like psoriasis, lupus, and pemphigus vulgaris. In pemphigus, for example, a genetic susceptibility combined with an environmental trigger (sometimes a chemical or medication) causes the immune system to attack healthy skin cells, producing painful blisters.
Sun damage and other environmental exposures cause lesions that range from sunburn to precancerous spots. Physical trauma, from a scrape to a burn, produces lesions as well. And hormonal changes, clogged pores, and genetic factors drive conditions like acne, skin tags, and birthmarks. Many common lesions have no single dramatic cause. They simply accumulate with age, sun exposure, and normal wear on the skin.
How Skin Lesions Are Diagnosed
Most lesions can be identified by appearance alone during a physical exam. A dermatologist evaluates the lesion’s type, color, texture, location on the body, and how it’s arranged relative to other spots. When the diagnosis isn’t clear from a visual exam, a biopsy is the next step.
There are three common biopsy techniques, and which one your doctor uses depends on the lesion’s size and depth. A shave biopsy scrapes a thin layer off the top of the skin and typically doesn’t require stitches. A punch biopsy uses a small circular cutting tool to remove a deeper core of tissue, including the top skin layers and some fat beneath. This one usually needs a stitch or two. An excisional biopsy removes the entire lesion along with a border of healthy skin around it, and it requires stitches to close. In all three cases, the tissue goes to a lab where it’s examined under a microscope.
Warning Signs Worth Watching
The vast majority of skin lesions are benign. But certain changes deserve a closer look, especially when it comes to ruling out skin cancer. The ABCDE rule, developed by the National Cancer Institute, outlines the features of early melanoma:
- Asymmetry: One half of the mole doesn’t match the other.
- Border: The edges are ragged, notched, or blurred, sometimes with pigment spreading into surrounding skin.
- Color: The spot contains multiple shades of brown, black, or tan, or has areas of white, gray, red, pink, or blue.
- Diameter: The spot is larger than 6 mm (about the size of a pencil eraser), though melanomas can sometimes be smaller.
- Evolving: The mole has visibly changed in size, shape, or color over weeks or months.
Beyond the ABCDE criteria, MD Anderson Cancer Center highlights several other red flags: a sore that doesn’t heal within two weeks, a spot that bleeds or repeatedly crusts over, and any lesion that itches, hurts, or looks noticeably different from every other mole on your body. In general, any new or changing spot that persists for two weeks or more is worth having evaluated.
Treatment Depends Entirely on the Cause
Because “skin lesion” is such a broad category, treatment ranges from doing nothing at all to surgery. A harmless mole or skin tag may need no treatment unless it’s cosmetically bothersome or in a spot where it gets irritated by clothing. Infectious lesions like bacterial boils or fungal patches clear with the appropriate antimicrobial treatment. Inflammatory conditions like eczema or psoriasis are managed with topical treatments to reduce the immune response in the skin.
Suspicious or precancerous lesions are typically removed, often with one of the biopsy techniques described above, so the tissue can be examined and the abnormal cells eliminated at the same time. For confirmed skin cancers, the approach depends on the type, size, and location, but removal of the affected tissue is the cornerstone. Recovery from most lesion removals is straightforward: a small wound that heals over days to weeks, sometimes with a minor scar.
The single most useful thing you can do for your skin is pay attention to it. Familiarity with your own spots, bumps, and marks makes it far easier to notice when something new appears or something old starts changing.