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. 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. Most small cuts start to heal within a few days.

A wound heals in stages that overlap with one another: 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. Platelets are tiny parts of blood that gather at an injury and help form the clot that works like a natural bandage, and they release chemical signals that attract more platelets. Hemostasis, the first stage, comes first, when vessels narrow and a clot forms, which stops the loss of blood and gives cells a framework to move across.

After the clot forms, immune cells such as neutrophils arrive to clear bacteria and dead material from the wound, and they do it by releasing proteases that break down damaged tissue. Neutrophils live only about two days at a wound, so once their work is done 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, and white blood cells in general fight germs in a wound and begin to repair it, so the cleanup crew also starts the rebuilding.

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 a few days, about two to five. A clear or pale yellow fluid that seeps from a healing wound helps clean it, which is quite different from yellow or green pus under or around a scab, because pus is a sign that the cut is infected. An infection can make a wound larger and slower to heal, so the difference between a helpful fluid and pus is worth knowing. A chronic wound, by contrast, 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, so there is real work going on beneath it, and the scab guards the cut in the meantime. 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, with no need for anyone to help it along.

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 not every cut makes a scab, since a cut that is kept moist and covered may not form one. As skin cells called 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, the best plan is to leave it alone and keep the cut covered while the skin underneath does its work.

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 to crawl through a wound they release enzymes called matrix metalloproteinases that dissolve damaged parts of the surrounding tissue, since cells can only migrate over living tissue. Meanwhile, 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. The tissue that fills a healing wound is called granulation tissue, and new blood vessels grow into it to bring oxygen and nutrients, which is why tissue where new vessels have grown often looks red, because of all the tiny blood vessels called capillaries.

Collagen is a tough, white protein fiber that the body makes to join the edges of a wound, and fibroblasts make the collagen that forms a scar, which works like a biological Band-Aid for damaged skin. Collagen is made quickly at first, but enzymes called collagenases break it down at the same time. Soon after wounding, making outpaces breaking down, so collagen builds up, and later the two become equal, so there is no net gain. Wound contraction starts about a week after wounding, when fibroblasts have turned into myofibroblasts, cells that are similar to smooth muscle cells. Myofibroblasts grip the surrounding tissue through adhesions called fibronexus links, and when the cells contract, they pull the tissue and reduce the wound size. A large wound can become 40 to 80% smaller after contraction, and contraction goes on even after the wound is fully covered by new skin.

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. The maximum strength of a scar is 80% of the strength of unwounded skin. In healthy skin, collagen fibers are woven like a basket, and how collagen is arranged shows how much a wound scars, since aligned collagen fibers lead to significant scarring. Scar tissue has no hair follicles, sweat or oil glands, or nerves, so a scar cannot do everything that normal skin does.

A scar is a quick patch that covers a wound fast, and speedy healing was an evolutionary advantage. Humans and other mammals mostly patch wounds with scar tissue, while some lizards, salamanders and fish can regrow lost body parts. Scarless healing happens in the tissues of mammal fetuses, and complete regeneration is limited to lower vertebrates such as salamanders. In adult humans, injured tissues are repaired by collagen deposition, remodeling and eventually a scar, although scientists first saw fetal skin heal without a scar 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 they can regrow hair follicles, oil glands, skin and cartilage in a hole punched in the ear. In the ear, that means hair follicles, oil glands, skin and cartilage all grow back instead of a scar forming. The spiny mouse result challenged the old idea that mammals can hardly replace injured 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, and it applies to every cut at every age: wash it, and tell a grown-up.

Researchers follow cells in a healing wound by tagging them with fluorescent markers and watching where they go. Stanford researchers used fluorescent markers to trace where scar-producing cells in mice come from. As of November 2026, Stanford researchers report that these cells arise from a group of fibroblasts that do not make engrailed-1 at first, and the gene flips on only when the animal is wounded. Scientists can find out what a cell type does by removing it, and in mice without engrailed-positive fibroblasts, scarring dropped sharply. As of November 2026, the researchers guessed the trigger might be mechanical, the force of the skin splitting apart, and in mice, fibroblasts under more tension produced more of a protein called YAP. In 2021 Stanford researchers reported that an eye-disease drug, verteporfin, blocked scar-making signals in mice, whose new skin grew back hair follicles and glands. Most scarring results come from mice. As of November 2026, a first small human trial of the eye drug verteporfin to reduce scars has just begun, with no results yet.

Words to know

• hemostasis: stopping the leak
• fibroblasts: collagen-making cells
• keratinocytes: skin-closing cells
• fluorescent: glowing
• verteporfin: an eye drug