Alopecia areata is caused by your own immune system attacking hair follicles, triggering sudden, patchy hair loss. Specifically, a type of white blood cell called a cytotoxic T cell infiltrates the tissue around hair follicles and disrupts their normal growth cycle. About 0.2% of the global population has the condition at any given time, but the lifetime risk is surprisingly high: roughly 32% for women and 17% for men worldwide.
The attack doesn’t destroy the follicle permanently, which is why hair can regrow. But what sets off the immune system in the first place involves a combination of genetic vulnerability, environmental triggers, and sometimes other autoimmune conditions working in the background.
The Immune Attack on Hair Follicles
Hair follicles normally enjoy a form of protection called “immune privilege,” meaning the immune system largely leaves them alone. In alopecia areata, that protection collapses. The follicle begins sending out stress signals, essentially molecular flags that attract the attention of immune cells that would otherwise ignore it.
The key players are a subset of killer T cells (CD8+ T cells carrying a receptor called NKG2D). These cells are both necessary and sufficient to trigger the condition, meaning without them, the hair loss doesn’t happen. Once they arrive at the follicle, they release a signaling molecule called interferon-gamma, which does two damaging things at once. It further strips away the follicle’s immune privilege, and it stimulates nearby cells to produce additional signals that recruit and sustain even more of the same killer T cells.
This creates a self-reinforcing loop. The immune attack generates the very conditions that fuel more immune attack. It’s why alopecia areata can progress quickly from a single small patch to widespread hair loss in some people, while in others the loop stays contained. The follicle itself isn’t destroyed during this process. It’s pushed into a dormant state, which is why regrowth remains possible even after prolonged episodes. For patchy alopecia areata, 30 to 50% of people experience spontaneous regrowth within the first 6 to 12 months without treatment.
Genetic Risk Factors
Alopecia areata runs in families, and the strongest genetic links involve a group of immune-system genes called HLA (human leukocyte antigen) genes. These genes control how your immune cells identify threats, and certain variations make it more likely that T cells will mistakenly target hair follicles.
Two specific gene variants, HLA-DRB1*04 and HLA-DRB1*16, are associated with increased risk. People carrying the DRB1*04:01 variant have roughly 64% higher odds of developing the condition compared to those without it. On the other hand, some HLA variants appear protective: DRB1*0301, DRB1*09, and DRB1*13 are each linked to lower risk, with DRB1*0301 cutting the odds by more than half.
These genes don’t guarantee you’ll develop alopecia areata. They set the stage. Most people with high-risk variants never lose a single patch of hair, because the genetic predisposition typically needs an environmental push to become active disease.
Environmental Triggers
For someone with genetic susceptibility, certain events can tip the immune system into attacking hair follicles. The most commonly reported triggers fall into two categories: psychological stress and viral infections.
Viral infections are a well-documented trigger. When your body fights a virus, it ramps up production of interferons, the same family of signaling molecules that drive the immune attack on hair follicles. COVID-19 has been linked to new onset, worsening, or recurrence of alopecia areata in previously infected patients. Epstein-Barr virus (the cause of mono) has also been associated with the condition, with hair loss appearing anywhere from one week to six months after infection. Hepatitis B is another documented trigger, with research showing that a protein on the virus’s surface shares structural similarities with receptors on human immune cells, potentially confusing the immune system into attacking the wrong target.
Psychological stress is harder to study in a controlled way, but it’s one of the most frequently self-reported triggers among patients. The proposed mechanism involves stress hormones altering immune function enough to break the hair follicle’s immune privilege in someone already genetically primed. Physical trauma or illness that puts significant strain on the body may work through similar pathways.
Connections to Other Autoimmune Conditions
If you have alopecia areata, you’re roughly twice as likely to have another autoimmune or immune-mediated condition compared to the general population. One large study covering 2007 to 2023 found that 16.1% of alopecia areata patients had at least one additional immune-mediated condition, versus 8.9% in a control group.
The most common overlapping conditions include atopic dermatitis (eczema), psoriasis, vitiligo, thyroid diseases like Hashimoto’s thyroiditis and Graves’ disease, rheumatoid arthritis, and lupus. Less common but still significantly elevated are inflammatory bowel disease, celiac disease, and pernicious anemia. This clustering suggests shared genetic and immune pathways across these conditions. Having one doesn’t mean you’ll develop others, but it does mean your immune system has a general tendency toward overactivity that can manifest in different organs.
How Alopecia Areata Is Recognized
The condition typically appears as round or oval patches of smooth, bare skin on the scalp, though it can affect the beard, eyebrows, eyelashes, or any body hair. The patches often appear suddenly, sometimes overnight or over a few days.
Dermatologists look for specific physical markers that distinguish alopecia areata from other types of hair loss. The most characteristic is “exclamation mark hairs,” short broken hairs that are thinner at the base than the tip, found at the edges of a bald patch. Black dots, which are pigmented hairs broken off right at the scalp surface, indicate active disease and correlate with severity. Yellow dots inside follicle openings represent a buildup of oil and skin debris in dormant follicles. When treatment is working, fine “vellus” hairs (very short, thin, often unpigmented) appear in the affected area, sometimes visible under magnification before they’re noticeable to the eye.
Treatment Options
Understanding the immune mechanism behind alopecia areata led directly to the development of targeted treatments. The self-reinforcing loop of T cell signaling depends on a family of enzymes called JAK (Janus kinase) enzymes, and blocking them can interrupt the cycle.
The first JAK inhibitor approved by the FDA for severe alopecia areata came in June 2022 (baricitinib), followed by a second in 2023 (ritlecitinib, also approved for patients 12 and older) and a third in 2024 (deuruxolitinib). These are oral medications that work by dampening the specific immune signals driving follicle attack. They represent a significant shift from older approaches like corticosteroid injections and topical immunotherapy, which suppress the immune response more broadly and have more variable results.
JAK inhibitors are currently approved for severe cases, meaning significant hair loss that hasn’t responded to other treatments. They require ongoing use, as the underlying autoimmune tendency remains even when hair regrows. For milder or patchy cases, many people regrow hair without any treatment at all, and corticosteroid injections into affected patches remain a common first-line option to speed recovery.