Alopecia areata is triggered when the immune system attacks hair follicles, but what sets off that attack varies from person to person. About 2% of the global population is affected, with a lifetime risk between 1.7% and 2.1%. The condition results from a combination of genetic vulnerability and environmental factors that tip the immune system into action against its own hair.
How the Immune System Turns on Hair Follicles
Hair follicles normally enjoy a form of immune protection. They exist in a kind of biological safe zone where the body’s immune cells leave them alone. In alopecia areata, that protection breaks down. When it does, immune cells that were previously kept at bay suddenly “see” proteins in the hair follicle that they interpret as foreign threats.
What follows is an inflammatory assault. Immune cells, particularly a type of white blood cell that kills targeted cells directly, swarm around the base of the hair follicle. Biopsies from affected skin show clusters of these immune cells surrounding follicles, along with elevated levels of the molecules that help the immune system identify targets. The follicle doesn’t die permanently, which is why regrowth is possible, but it’s forced into a dormant state and stops producing hair.
The collapse of the follicle’s immune protection can be set off by several signals: a surge in inflammatory molecules, a drop in the local chemicals that normally suppress immune activity, or activation of nearby cells that amplify the immune response. Once one patch loses protection, the inflammatory signaling can sometimes spread to neighboring follicles, which explains why patches can expand or multiply.
Genetics Load the Gun
You don’t inherit alopecia areata directly, but you can inherit the susceptibility. Researchers have identified dozens of genes linked to the condition, most of them involved in immune regulation. A large cluster belongs to the HLA gene family, which controls how the immune system distinguishes your own tissues from foreign invaders. Variations in these genes can make the immune system more prone to misidentifying hair follicle proteins as threats.
Beyond HLA genes, other associated genes are involved in inflammation, immune cell signaling, and the regulation of specific types of white blood cells. Having a first-degree relative with alopecia areata significantly raises your risk, though many people with the genetic profile never develop the condition. Genetics creates the vulnerability; something else has to pull the trigger.
Stress and the Cortisol Connection
The link between psychological stress and hair loss flares is more than anecdotal. Research from Harvard’s Stem Cell Institute identified a specific pathway: chronic stress raises levels of cortisol, the body’s primary stress hormone, which acts on a cluster of cells beneath the hair follicle called the dermal papilla. Under high cortisol, these cells stop releasing a signaling molecule that normally activates hair follicle stem cells, keeping follicles locked in a resting phase and unable to grow new hair.
This creates what researchers describe as an “adrenal gland to hair follicle axis.” Your body already uses cortisol to regulate hair cycling under normal conditions, with higher cortisol levels naturally keeping some follicles dormant. Chronic stress amplifies this existing system, making it substantially harder for stem cells to enter the growth phase. For someone genetically prone to alopecia areata, this stress-driven suppression can coincide with or worsen the immune attack on follicles.
Viral Infections as a Catalyst
Infections are among the most commonly reported triggers preceding a first episode or a flare. Specific viruses linked to alopecia areata onset include Epstein-Barr virus (the cause of mono), hepatitis B and C, and influenza strains including swine flu. COVID-19 infection has also been identified as a trigger.
The likely mechanism is that a viral infection activates the immune system broadly. In someone with genetic susceptibility, that heightened immune state can spill over into an autoimmune response against hair follicles. The infection itself resolves, but the misdirected immune attack persists. Many people report their first patch of hair loss appearing weeks after recovering from an illness, which aligns with the timeline of the immune system ramping up and then failing to fully stand down.
Vaccinations and Immune Activation
Vaccines, which work by stimulating the immune system, have been documented as occasional triggers. A search of the CDC’s Vaccine Adverse Event Reporting System found 67 cases of alopecia areata associated with Pfizer or Moderna COVID-19 vaccines. Cases have also been reported after tetanus, Japanese encephalitis, influenza, and routine childhood vaccinations.
In a case series of nine patients who developed alopecia areata after COVID-19 vaccination, six experienced hair loss within one month of completing their vaccine series. Across multiple reports, 84% of post-vaccination hair loss appeared within one month. This is a rare occurrence relative to the hundreds of millions of vaccine doses administered, and the mechanism likely parallels infection-triggered cases: the immune stimulation from the vaccine, in a genetically predisposed person, tips the balance toward autoimmune activity at the hair follicle.
Gut Health and Bacterial Imbalance
The gut microbiome, the community of bacteria living in your digestive system, appears to play a role. Research published in the Journal of Investigative Dermatology found that mice who developed alopecia areata had an overgrowth of a specific gut bacterium (Ligilactobacillus murinus) before their hair loss appeared, while mice protected from the condition had lower levels of that same bacterium.
In human patients, the findings were consistent. Stool samples from people with alopecia areata showed striking differences in gut bacterial composition compared to healthy controls, with overgrowth of bacteria from the Lachnospiraceae and Bacteroidaceae families. This is still an emerging area, but it suggests that the state of your gut bacteria can influence whether your immune system stays balanced or tips toward attacking your own tissues.
Thyroid Disease and Other Autoimmune Conditions
Having one autoimmune condition raises the odds of developing another. Thyroid disorders are particularly common in people with alopecia areata. A meta-analysis found that 13.3% of alopecia areata patients had thyroid dysfunction, and individual studies have reported rates as high as 25% for comorbid thyroid issues, with hypothyroidism being the most frequent. Other associated autoimmune conditions include vitiligo (loss of skin pigmentation) and type 1 diabetes.
This clustering isn’t coincidental. The same genetic variants that predispose someone to alopecia areata overlap with those linked to thyroid autoimmunity and other conditions. If you develop alopecia areata, screening for thyroid function is worthwhile, since thyroid problems are treatable and can independently affect hair health and energy levels.
Vitamin D Deficiency
People with alopecia areata are nearly five times more likely to be deficient in vitamin D than the general population. A meta-analysis in the Journal of the American Academy of Dermatology confirmed both a higher rate of deficiency and significantly lower average vitamin D levels in affected patients.
Vitamin D plays an active role in keeping the immune system balanced. It suppresses the types of immune cells most responsible for autoimmune attacks while boosting regulatory cells that prevent the immune system from overreacting. Whether low vitamin D directly triggers flares or simply reflects a shared underlying immune dysfunction isn’t fully settled, but maintaining adequate levels supports the arm of the immune system that keeps autoimmune activity in check.
Physical Trauma to the Scalp
Localized injury to the skin can trigger new patches of hair loss at the site of trauma, a phenomenon known as the Koebner response. This has been documented after scalp injuries, surgical scars, and even diagnostic hair-pulling tests performed in dermatology offices. In one case series, new patches appeared within one to seven days of a minor scalp procedure, suggesting that physical disruption can rapidly activate the immune attack in skin that was previously unaffected.
This has practical implications. Aggressive styling, tight hairstyles that pull on follicles, and rough handling of areas near existing patches can potentially provoke new spots of hair loss. It also means that some diagnostic procedures performed on the scalp carry a small risk of triggering the very condition they’re investigating.