Because your skin keeps germs out of your body, any cut, scrape or scratch that breaks it is a wound, and the first rule at every age is a short one: wash it, and tell a grown-up. Hands should be washed with soap and water before a cut is cleaned, and the cut itself is rinsed with clean water to wash the dirt away. Washing it with mild soap and water soon after it happens helps stop infection, while a bandage covers a small cut to keep it clean while it heals. Even small cuts and scrapes need to be cleaned and covered so they do not get infected.

A wound heals in stages that overlap: the leak is stopped, germs and debris are cleared, new tissue is built, and the repair is strengthened. Within the first few minutes of an injury, platelets in the blood begin to stick to the injured site, and fibrin then forms a mesh that acts like glue and binds the platelets to each other. Hemostasis comes first, when vessels narrow and a clot forms, which stops the loss of blood and gives cells a framework to move across. Until collagen is deposited, a fibrin-fibronectin plug is the main structural support for the wound, and repair cells use that plug as a matrix to crawl across.

Platelets at an injury release chemical signals that attract more platelets, and then immune cells such as neutrophils arrive to clear bacteria and dead material from the wound. Neutrophils clean a wound by releasing proteases that break down damaged tissue, and since they live only about two days at a wound, they die and are engulfed and broken down by macrophages. Blood cells called monocytes enter a wound from the bloodstream through blood vessel walls and mature into macrophages there. Low oxygen around macrophages stimulates them to make factors that induce and speed up the growth of new blood vessels.

In the first few days a cut can be swollen, red and sore, which is the immune system protecting the wound, and this stage lasts about two to five days. A clear or pale yellow fluid that seeps from a healing wound helps clean it, but yellow or green pus under or around a scab is a sign that the cut is infected. An infection can make a wound larger and slower to heal. A chronic wound is one that fails to move through the normal phases of healing in an orderly and timely manner, and such wounds often stall in the inflammation phase.

A scab is a hard crust that forms over a cut or scrape, and it is a dried clot. It forms within seconds or minutes after the skin is hurt, and it works like a little shield that helps keep germs out of the cut. New skin cells appear underneath the scab, and the scab stays in place until they do. A scab can itch while the skin under it heals, but scratching it can break the new skin, and picking a scab makes a cut heal more slowly. When the skin underneath has healed, the scab falls off by itself.

A scab protects a wound, but it can also slow healing, because wounds with scabs take longer to heal than wounds kept moist and covered. Keeping a cut moist with petroleum jelly helps it heal faster than leaving it dry, and a cut that is kept moist and covered may not form a scab at all. As keratinocytes migrate they move over granulation tissue but stay underneath the scab, which separates the scab from the tissue below. A moist environment helps them travel, since a dry one leads to a bigger, tougher scab, so if a scab does form, leave it alone.

Skin cells called keratinocytes come from the wound edges and from hair follicles, sweat glands and oil glands. Before they start to move, keratinocytes change shape, becoming longer and flatter and putting out ruffle-like edges, and the growing sheet of skin cells is often called the epithelial tongue. These cells crawl across a wound, but they can only travel over living tissue, so a deep wound must first be filled with granulation tissue. Skin cells behind the migrating edge divide to supply more cells, and this growth runs 17 times faster than in normal tissue.

About two or three days after a wound forms, fibroblasts begin to enter it, and by the end of the first week they are the main cells in the wound. Early in healing, a wound is held together by type III collagen and fibronectin, which are produced in large amounts from about 10 hours to 3 days after injury, depending mainly on wound size. During maturation the stronger type I collagen replaces them. Fibroblasts and sprouting blood vessels react differently to low oxygen, since low oxygen speeds up vessel sprouting but slows fibroblast growth, and too little oxygen can lead to excessive, fibrotic scarring.

Wound contraction starts about a week after wounding, when fibroblasts have turned into myofibroblasts, cells similar to smooth muscle cells. Contraction can last for several weeks and goes on even after the wound is fully covered by new skin, and a large wound can become 40 to 80% smaller after contraction. In the maturation phase, the first disorganized collagen fibers are rearranged, cross-linked and lined up along tension lines. Collagen reaches about 20% of its tensile strength after three weeks and about 80% after 12 months, and the scar loses its red look because blood vessels that are no longer needed are removed by programmed cell death.

In healthy skin, collagen fibers are woven like a basket. In a scar, fibroblasts lay the collagen down quickly in parallel lines, so the tissue is thicker, weaker and less flexible. Fibroblasts make the collagen that forms a scar, which works like a biological Band-Aid for damaged skin, and a scar is a quick patch that covers a wound fast. Speedy healing was an evolutionary advantage, and humans and other mammals mostly patch wounds with scar tissue, while some lizards, salamanders and fish can regrow lost body parts.

Some warning signs mean a grown-up should step in. Spreading redness, more pain, swelling or pus are signs of infection, and a grown-up should call a doctor. Deep, large or dirty cuts need more care than small clean ones, and a doctor may need to look at them. Soil or dirt that gets into a wound can cause tetanus, so after a dirty or rusty cut a person should check that their tetanus shot is up to date. A wound that comes open again, or whose stitches come out too soon, needs a doctor, and black edges around a wound can be a sign of dead tissue. Even then the first rule stays the same: wash it, and tell a grown-up.

Fetal skin can heal without a scar, and scientists first saw this in lambs in the early 1970s. African spiny mice can shed patches of skin to escape a predator, then regrow skin and hair follicles instead of a scar, and their skin tears very easily, which may help them escape the grip of a predator. Spiny mice can also regrow hair follicles, oil glands, skin and cartilage in a hole punched in the ear. Scientists reported that removing macrophages from a salamander made its limb fail to regenerate, and the salamander made a repair, or scarring, response instead.

In mice, a fibroblast type marked by the gene engrailed-1 builds most scar tissue, and it makes scar tissue only when the skin is under stress. As of November 2026, tension on a healing wound is known to make these fibroblasts in mice produce more of a signal protein called YAP, which researchers tied to scarring. Scientists can find out what a cell type does by removing it. In mice without engrailed-positive fibroblasts, scarring dropped sharply.

Words to know

• fibroblasts: collagen-making cells
• collagen: tough white fiber
• cartilage: bendy ear tissue
• follicles: hair roots
• tension: pulling force