Alopecia areata is caused by your own immune system mistakenly attacking hair follicles. Specifically, certain white blood cells identify proteins in your hair follicles as foreign threats and launch an inflammatory assault that forces hair out of its growth phase prematurely. The result is patchy, sometimes widespread hair loss that can be unpredictable and cyclical. What makes the condition complex is that no single factor pulls the trigger. It takes a combination of genetic vulnerability, an immune system glitch, and often an environmental spark to set it off.
How the Immune System Turns on Hair Follicles
Hair follicles normally enjoy a form of biological protection called immune privilege. This means they’re shielded from immune surveillance, similar to the way the eyes and parts of the brain are. In alopecia areata, that protection breaks down. Once it collapses, immune cells can suddenly “see” proteins inside the follicle that were previously hidden, and they treat those proteins as dangerous invaders.
What follows is a rapid immune response. Antigen-presenting cells alert the rest of the immune system, and two types of T cells (CD4+ helper cells and CD8+ killer cells) swarm in and around the follicle. Pathologists who examine scalp biopsies from people with alopecia areata describe this dense cluster of immune cells as a “swarm of bees.” The infiltrate isn’t just T cells. It also includes natural killer cells, mast cells, and other inflammatory cells, all contributing to the attack.
The killer T cells are especially important. They produce a signaling molecule called interferon-gamma, which in turn recruits more inflammatory cells, creating a self-reinforcing feedback loop. This loop forces hair out of its active growth phase and into a resting and shedding phase far too early. The follicle itself isn’t destroyed, which is why regrowth remains possible, but as long as the immune attack continues, new hair can’t emerge or survive.
Regulatory T cells, which normally act as peacekeepers and prevent the immune system from going too far, appear to be deficient or dysfunctional in people with alopecia areata. Fewer of these cells are found around affected follicles, which helps explain why the immune response spirals rather than self-correcting.
The Genetic Foundation
Alopecia areata runs in families, though the inheritance pattern is not straightforward. First-degree relatives of someone with the condition (siblings, parents, children) have a meaningfully higher risk of developing it compared to the general population. But having a relative with alopecia areata doesn’t guarantee you’ll get it, and many people develop it with no family history at all.
The genes most strongly linked to the condition belong to a family called the human leukocyte antigen (HLA) complex. These genes help your immune system distinguish your own cells from foreign ones. Certain variations in HLA genes appear to make the immune system more likely to misidentify hair follicle proteins as threats. Beyond the HLA complex, researchers have identified dozens of other associated genes involved in inflammation, immune signaling, and the activation of natural killer cells. This wide genetic footprint helps explain why the condition varies so much from person to person in severity and course.
Environmental Triggers That Set It Off
Genetics loads the gun, but something in the environment typically pulls the trigger. Viral infections are among the most well-documented sparks. Initial episodes have been linked to Epstein-Barr virus, hepatitis B and C, and swine flu. There is also evidence that COVID-19 infection can trigger the condition, likely by ramping up immune activity to a level that breaks down the follicle’s protective shield.
Psychological stress is another commonly reported trigger, though its role is harder to pin down scientifically. What’s clearer is that stress hormones can influence immune function in ways that may tip a genetically susceptible person over the edge. Iron deficiency has also been identified as a risk factor, possibly because nutrient-depleted follicles are more vulnerable to immune disruption.
Types and How the Condition Progresses
Alopecia areata typically starts as one or more coin-sized, round or oval patches of hair loss on the scalp. This patchy form is the most common and, for many people, is where it stays. In some cases, though, it progresses further:
- Alopecia totalis involves complete hair loss across the entire scalp.
- Alopecia universalis extends to the entire body, including eyebrows and eyelashes.
- Ophiasis causes hair loss in a band pattern along the sides and back of the head, and tends to be more resistant to treatment.
- Diffuse alopecia areata causes sudden, widespread thinning across the scalp that can be difficult to distinguish from other types of hair loss.
Hair loss and regrowth in all forms can be unpredictable and cyclical. Some people experience a single episode and regrow hair that never falls out again. Others go through repeated cycles over years. Importantly, regrowth can happen even after many years of severe or widespread loss, because the immune attack damages the hair cycle but does not permanently destroy the follicle itself.
Linked Autoimmune Conditions
Because alopecia areata is fundamentally an immune system disorder, it frequently overlaps with other autoimmune and inflammatory conditions. An estimated 39% of people with alopecia areata also have atopic dermatitis (eczema), making it the most common companion condition. There is a 16% increased risk of also having vitiligo, lupus, or thyroid disease. Rates of psoriasis and rheumatoid arthritis are also roughly double those seen in the general population.
This clustering suggests that the same genetic and immune vulnerabilities that cause alopecia areata can simultaneously affect other parts of the body. If you’ve been diagnosed with alopecia areata, screening for thyroid problems in particular is common practice, since thyroid autoimmunity is one of the most frequent overlaps.
How Treatments Target the Cause
Understanding the immune mechanism behind alopecia areata has led to targeted treatments that interrupt the specific signaling pathway driving the attack. The key pathway involved is called JAK-STAT, which acts like a relay system: inflammatory signals from T cells bind to receptors on the cell surface, activating a chain of proteins inside the cell that ultimately turns on genes responsible for more inflammation. Blocking this relay can quiet the immune assault on hair follicles.
Three JAK inhibitors are currently FDA-approved in the United States for adults with severe alopecia areata: baricitinib, ritlecitinib, and deuruxolitinib. These medications are taken by mouth and work by dampening the overactive immune response rather than stimulating hair growth directly. For many patients, this means hair regrowth becomes possible once the immune system stops interfering with the normal hair cycle. They are not cures. Hair loss can return if the medication is stopped, because the underlying genetic and immune susceptibility remains.
Older approaches like corticosteroid injections into the scalp are still used for patchy disease. They work by locally suppressing immune activity around the follicle. For milder cases, topical treatments that reduce inflammation at the skin surface remain a first option. The choice depends largely on how much hair has been lost and how rapidly the condition is progressing.