Healing refers to the process of repairing damaged tissues, organs, and the body as a whole so that normal functions can return. Medicine involves the way cells in the body repair and replace damaged or dead tissue. This replacement can occur in two ways: regeneration, where new cells grow to replace damaged cells and create tissue similar to the original, or repair, where injured tissue is replaced by scar tissue. Most organs use both methods together to heal.

In surgery, healing is often called recovery. After surgery, recovery was once thought to mean simply restoring normal function and being ready to leave the hospital. More recently, recovery has been described as a process that requires energy to reduce physical symptoms, improve emotional well-being, regain abilities, and return to daily activities.

Healing is also discussed in the context of grieving, which is the process of dealing with loss.

In psychiatry and psychology, healing is the process of helping individuals overcome mental health issues, such as neuroses or psychoses, so they can live a normal or satisfying life without being overwhelmed by mental health challenges. This process may include therapy, medicine, or other methods like traditional spiritual healing.

Regeneration

For an injury to heal through regeneration, the type of cell that was damaged must be able to make copies of itself. Cells also need a collagen structure to help them grow. Most cells are surrounded by either a basement membrane or a collagen network created by fibroblasts, which guide the cells as they grow. Ischemia and most toxins do not harm collagen, so it remains intact even when the cells around it are dead.

Acute tubular necrosis (ATN) in the kidney is an example of complete healing through regeneration. ATN happens when the epithelial cells lining the kidney are destroyed by a lack of oxygen, such as in hypovolemic shock (when blood flow to the kidneys is severely reduced), or by toxins like certain antibiotics, heavy metals, or carbon tetrachloride.

Although many epithelial cells are dead, the damage is often patchy, meaning some epithelial cells remain alive. Additionally, the collagen structure of the kidney’s tubules stays completely intact.

The surviving epithelial cells can make copies of themselves and use the basement membrane as a guide to regrow. Once regeneration is complete, the kidney returns to normal, and the damage cannot be seen even under a microscope.

When cells that cannot regenerate (such as neurons) are injured, healing must occur through repair. Similarly, damage to the collagen network (for example, by enzymes or physical injury) or its complete collapse (as in an infarct) also requires healing through repair.

Genetics

Many genes help the body heal wounds. For example, the P21 gene helps mammals heal wounds on their own. In some mammals, like mice, this gene allows wounds to heal without leaving scars. Another gene called LIN28 is involved in wound healing, but it is inactive in most mammals. Additionally, two proteins named MG53 and TGF beta 1 are important for the healing process.

Wound healing

When a cut or wound occurs, the body starts a process called wound healing. This process happens in four steps: clot formation, inflammation, proliferation, and maturation.

The first step is clot formation. This stops bleeding and helps prevent infection by bacteria, viruses, and fungi. After clotting, neutrophils (a type of white blood cell) enter the wound area between 3 to 24 hours after the injury. Around 24 to 48 hours later, epithelial cells begin to divide.

During the inflammation phase, macrophages and other phagocytic cells destroy bacteria, remove damaged tissue, and release chemical signals. These signals include growth hormones that help fibroblasts, epithelial cells, and endothelial cells move to the wound site and divide.

In the proliferation phase, immature granulation tissue forms. This tissue contains fibroblasts, which produce type III collagen. This collagen fills the space left by the wound. Granulation tissue grows from the edges of the injury toward the center, like a wave.

As granulation tissue matures, fibroblasts produce less type III collagen and become thinner. They start making stronger type I collagen. Some fibroblasts turn into myofibroblasts, which have a type of protein called actin found in smooth muscle. This allows them to contract and shrink the wound.

During the maturation phase, extra blood vessels in the granulation tissue are removed through a process called apoptosis. Type III collagen is mostly replaced by type I collagen. The collagen fibers, which were once disorganized, become tightly linked and arranged along tension lines. This phase can last up to a year or more. Eventually, a scar made of collagen and containing a few fibroblasts remains.

Tissue damaged by inflammation

After tissue is damaged by inflammation, such as during a bacterial infection, healing occurs in four stages.

In the recall stage, the adrenal glands produce more cortisol, which stops the production of inflammatory chemicals and reduces inflammation.

During the resolution stage, macrophages (a type of white blood cell) remove pathogens and damaged tissue. They also remove old red blood cells from the injured area. If not all damaged cells and pathogens are cleared, inflammation may return. Two types of macrophages, M1 and M2, are important here. M1 macrophages contribute to inflammation, while M2 macrophages help repair tissue. The ability of these cells to change between M1 and M2 roles affects whether inflammation continues or healing occurs.

In the regeneration stage, blood vessels are repaired, and new cells grow at the damaged site. These new cells are similar to the ones that were damaged and removed. Some cells, like neurons and muscle cells in the heart, take longer to recover.

In the repair stage, new tissue forms. This process requires a balance between anti-inflammatory and pro-inflammatory chemicals called eicosanoids. Anti-inflammatory eicosanoids, such as lipoxins, epi-lipoxins, and resolvins, help release growth hormones that support tissue repair.