Keloid scars form when the body’s wound-healing process goes into overdrive, producing far more collagen than needed and building up thick, raised tissue that grows beyond the edges of the original injury. Unlike normal scars, which gradually flatten and fade, keloids keep expanding into healthy surrounding skin, sometimes for months or years after the initial wound has closed. What drives this overreaction is a combination of genetics, an imbalanced immune response, and specific physical triggers.
How Normal Healing Goes Wrong
Every wound triggers a repair sequence: inflammation clears debris, new tissue fills the gap, and collagen fibers rebuild the skin’s structure. In keloid-prone individuals, this process never properly shuts off. The fibroblasts, the cells responsible for laying down collagen, become hyperactive. They multiply faster than normal and produce collagen at an elevated rate, creating a dense, disorganized mass of scar tissue.
Part of the problem is mechanical. Fibroblasts can sense the stiffness of the tissue around them. When the structure of the healing wound doesn’t match what the cells expect, they respond by ramping up collagen production even further, essentially trying to “fix” a mismatch that their own activity keeps making worse. This feedback loop is one reason keloids are so persistent and difficult to treat.
Several chemical signals are also abnormally elevated in keloid tissue. A growth factor called TGF-beta, which normally helps coordinate wound repair, is overexpressed alongside its receptors. So are signals that promote blood vessel growth and general cell proliferation. The result is a wound-healing environment stuck in a permanent “on” state, with no mechanism to wind things down.
The Role of the Immune System
Keloids aren’t just a collagen problem. They also show high levels of immune cell infiltration. Research using single-cell analysis of keloid tissue has found that a particular type of immune cell, called M2 macrophages, dominates the keloid environment. These cells promote tissue building and fibrosis rather than the inflammation-clearing work of their counterparts (M1 macrophages). In practical terms, the immune system in a keloid is tilted toward encouraging more scar tissue rather than breaking it down.
The local chemical environment reflects this imbalance. Pro-fibrotic signaling molecules like IL-4, IL-6, IL-10, IL-13, and TGF-beta are markedly elevated inside keloids, while molecules that normally suppress collagen production and slow fibroblast growth are reduced. This creates a microenvironment that actively sustains keloid growth long after the original injury has healed. It’s this chronic, self-reinforcing process that distinguishes keloids from ordinary scars or even hypertrophic scars, which eventually stabilize on their own.
Genetics and Family History
The strongest predictor of keloid formation is family history. Keloid susceptibility runs in families, typically following an autosomal dominant pattern, meaning you only need to inherit the trait from one parent to be at risk. However, the genetics are complex. Penetrance is incomplete, so not everyone who carries the relevant genes will develop keloids, and expressivity varies, meaning family members may develop keloids of different sizes or in different locations. Some families show autosomal recessive or even X-linked inheritance patterns, pointing to multiple genes being involved.
Genome-wide studies in Japanese and Chinese populations have identified several genetic regions associated with keloid risk, including variants on chromosomes 1 and 15. Specific immune-related gene variants (HLA alleles) have also been linked to keloids in both European and Chinese populations, reinforcing the connection between immune regulation and scar formation. Twin studies further support the genetic basis: keloid occurrence is significantly higher among twins than in the general population.
Ethnicity and Demographics
Keloid prevalence varies dramatically across ethnic groups. People of African descent have the highest rates, estimated at 4 to 6 percent of the population, with some studies reporting rates as high as 16 percent in certain African communities. People of Asian and Hispanic descent have moderate susceptibility. White populations have the lowest prevalence, recorded as low as 0.09 percent in one English study. These differences are consistent with the genetic basis of the condition and suggest that multiple gene variants contributing to keloid risk are more common in certain populations.
Keloids most commonly appear between puberty and age 30, a period of higher hormonal activity. They can develop at any age, but are rare in young children and older adults.
Triggers That Start the Process
In someone with genetic susceptibility, almost any form of skin injury can trigger a keloid. Common triggers include:
- Piercings, especially earlobe piercings, which account for over 70 percent of head and neck keloids
- Acne, particularly cystic acne on the chest, jawline, or shoulders
- Surgical incisions, including cosmetic procedures
- Burns, even relatively minor ones
- Vaccinations and injections
- Tattoos
- Insect bites, scratches, and ingrown hairs
The threshold can be remarkably low. Some keloids develop from injuries so minor the person doesn’t remember them. In rare cases, keloids appear to form spontaneously, though it’s likely an unnoticed micro-injury was the initial trigger.
Body Location Matters
Not all skin is equally susceptible. Keloids strongly favor areas where the skin is under constant mechanical tension. The anterior chest, shoulders, upper back, and the front of the neck are classic high-risk zones. Wounds that cross natural skin tension lines are especially prone to abnormal scarring.
On the head and neck, the earlobe is by far the most common site, accounting for about 53 percent of ear keloids. The areas around the ears, along the jawline and beard region, and under the chin are also frequently affected. By contrast, the palms, soles, and eyelids almost never develop keloids, likely because of differences in skin structure and tension.
Why Keloids Keep Growing
One feature that separates keloids from hypertrophic scars is their refusal to stop. Hypertrophic scars stay within the boundaries of the original wound and often improve over one to two years. Keloids, by definition, extend beyond the wound margins and invade healthy tissue. They can continue growing for years, and they recur frequently after treatment.
Surgical removal alone leads to recurrence roughly half the time. One study found that 51 percent of surgically excised keloids returned when no additional treatment was used. This is because cutting out a keloid creates a new wound in skin that is already predisposed to abnormal healing, restarting the same cascade of overactive fibroblasts and imbalanced immune signals. Combining surgery with additional therapies significantly reduces recurrence, which is why excision alone is rarely recommended.
The persistence of keloids reflects the underlying biology: as long as the genetic susceptibility exists and the immune microenvironment remains tilted toward fibrosis, any disruption to the skin carries the risk of triggering the same overactive repair process all over again.