Skin cancer starts when DNA damage causes skin cells to grow out of control, forming tumors that can destroy surrounding tissue and, in serious cases, spread to other organs. It is the most common type of cancer, with melanoma alone expected to account for 112,000 new diagnoses in the U.S. in 2026. What it does to your body depends on the type, how deep it grows, and whether it stays local or travels elsewhere.
How Skin Cancer Starts
Your skin’s outer layer, the epidermis, constantly produces new cells at its base and pushes older ones toward the surface. Skin cancer begins when UV radiation from the sun or tanning beds damages the DNA inside these cells. UVB rays cause specific types of DNA errors called cyclobutane pyrimidine dimers, which lead to characteristic mutations as the cell tries to copy its damaged genetic code. UVA rays penetrate deeper into the skin, reaching the pigment-producing cells (melanocytes) at the base of the epidermis, though their mutagenic effect on those cells appears surprisingly limited compared to UVB.
These DNA changes give cells new, harmful instructions: multiply faster than normal, and don’t die when you’re supposed to. The result is a growing mass of abnormal cells that crowds out and destroys healthy tissue. Depending on which cell type is affected, the cancer takes one of three main forms: basal cell carcinoma (from the deepest cells of the epidermis), squamous cell carcinoma (from the flatter cells closer to the surface), or melanoma (from pigment-producing melanocytes).
What Each Type Does to Your Body
Basal Cell Carcinoma
This is the most common and least aggressive form. It grows slowly and rarely spreads beyond the original site. But “least aggressive” doesn’t mean harmless. Left alone, basal cell carcinoma invades and destroys the tissue around it, eating into skin, cartilage, and sometimes bone. On the face, that can mean damage to the nose, ears, or eye area. It typically appears as a pearly or waxy bump, a flat scar-like lesion, or a sore that repeatedly bleeds, crusts over, and never fully heals.
Squamous Cell Carcinoma
Squamous cell carcinoma is more aggressive than basal cell. It can cause painful or itchy skin lesions and tends to grow faster. While most cases stay localized if caught early, this type has a real capacity to invade deeper tissue and spread to lymph nodes. It often shows up as a firm red nodule or a flat lesion with a scaly, crusted surface.
Melanoma
Melanoma is the most dangerous form because of its ability to spread. It usually isn’t painful in its early stages but may itch or bleed as it progresses. It develops in pigment-producing cells, which is why it often appears as a dark or multicolored spot. The ABCDE rule helps identify suspicious changes: asymmetry (one half doesn’t match the other), irregular borders (ragged or blurred edges), uneven color (mixtures of brown, black, tan, white, red, or blue), diameter larger than about 6 millimeters (roughly the size of a pencil eraser), and evolution (a spot that changes in size, shape, or color over weeks or months).
How It Spreads
The most serious thing skin cancer can do is metastasize, meaning it sends cancer cells to distant parts of the body. Melanoma is particularly prone to this. Cancer cells break away from the original tumor and travel through the lymphatic system or bloodstream to establish new tumors in the lungs, liver, brain, or bones.
Research from the National Cancer Institute has revealed why this process is so effective. Melanoma cells tend to spread first to nearby lymph nodes, and that stopover turns out to be strategically important. While circulating in the lymphatic system, the cells absorb a fatty acid called oleic acid, which gets incorporated into their outer membranes. This protects them from a natural form of cell death called ferroptosis, which normally kills cancer cells when they enter the bloodstream. Essentially, the lymph node visit makes the cells more durable. As one researcher described it, the melanoma cells “load up on oleic acid in the lymph, and then once they go into the blood, they’re bulletproof.”
How It Hides From Your Immune System
Your immune system is designed to find and kill abnormal cells, and T cells are the primary soldiers in that fight. They recognize markers on the surface of cancer cells and attack. Skin cancer, particularly melanoma, uses several strategies to disable this defense.
One approach is camouflage. Tumor cells reduce the surface markers that T cells use to identify them, making themselves harder to detect. Another is chemical warfare: the tumor secretes substances that suppress immune cell activity in the surrounding tissue, essentially creating a hostile environment for the very cells trying to fight it. The tumor also recruits regulatory immune cells that normally prevent the immune system from attacking healthy tissue, turning them into shields that protect the cancer instead.
Perhaps the most well-understood trick involves a molecular handshake between a protein on the tumor’s surface and a receptor on T cells. When they connect, the T cell receives a “stand down” signal, stops multiplying, and eventually becomes exhausted and inactive. This mechanism is so important that modern immunotherapy treatments work by blocking it, freeing T cells to resume their attack.
What Progression Looks Like
Skin cancer is staged based on three factors: the size and depth of the tumor, whether it has reached nearby lymph nodes, and whether it has spread to distant organs. Higher numbers mean more advanced disease.
In early stages, the cancer is confined to the skin. You might notice a spot that looks unusual, bleeds, or doesn’t heal. At this point, the damage is local. As the tumor grows deeper, it can invade underlying structures like fat, muscle, cartilage, or bone. If cancer cells reach the lymph nodes, the disease is regional. And once it appears in distant organs, it’s classified as stage IV.
The difference between catching it early and catching it late is stark. For melanoma that’s still localized to the skin (stages I and II), the five-year survival rate is 97.6%. Once it reaches nearby lymph nodes (stage III), that drops to 60.3%. If it has spread to distant organs (stage IV), the five-year survival rate falls to 16.2%. Basal and squamous cell carcinomas have much higher survival rates overall, but they can still cause significant tissue destruction if ignored.
Symptoms You Might Feel
Skin cancer is often painless in its early stages, which is part of what makes it dangerous. Many people expect cancer to hurt, but the first signs are usually visual: a new spot, a mole that changes, or a sore that won’t heal. As it progresses, some types cause itching, tenderness, pain, or a burning sensation. Squamous cell carcinoma in particular can be painful or itchy. Melanoma may begin to itch or bleed as it grows.
Late-stage symptoms depend on where the cancer has spread. Melanoma that reaches the lungs may cause shortness of breath. In the brain, it can cause headaches or neurological changes. In bones, persistent pain. These symptoms reflect the cancer’s ability to disrupt whatever organ it colonizes, not just the skin where it started.