How Does Alopecia Areata Develop?

Alopecia areata is the second most common cause of hair loss after pattern (androgenetic) hair loss. The condition affects about one to two percent of the population and occurs equally in women and men. Especially notable: patchy hair loss can appear suddenly and without warning, with coin-sized bald spots on the scalp as the characteristic sign.

This article looks more closely at the scientific facts on alopecia areata. The lifetime probability of developing this form of alopecia at least once is about 2% (lifetime incidence). It is also important to know that in 40% of patients the disease first appears before age 20—and in about 85% alopecia areata shows by age 40. The good news: in roughly one third of those affected, hair on the bald patches grows back on its own within a year.

What Is Alopecia Areata?

Patchy hair loss, or **alopecia areata**, is a chronic, immune-mediated disease marked by sudden-onset hair loss. Unlike other forms of hair loss, in alopecia areata the hair root remains intact, so follicles stay visibly preserved. It is therefore a non-scarring form of hair loss.

Definition and Distinction From Other Alopecias

Alopecia areata is considered an autoimmune disease that occurs in genetically predisposed people after exposure to unknown environmental triggers. Typically, small round hairless patches appear, at whose edges short broken hairs resemble exclamation marks. These so-called "exclamation-mark hairs" or "cadaver hairs" are an important diagnostic feature.

Unlike other alopecias such as tinea capitis (fungal infection), trichotillomania (self-induced hair loss), lupus, or secondary syphilis, alopecia areata lacks inflammatory signs such as redness, scaling, or itching. At the active edge, hairs can also be pulled out in clumps without pain.

Frequency and Affected Age Groups

The lifetime probability of developing alopecia areata at least once is about 2% (lifetime incidence). The disease affects men and women equally and occurs in all age groups and ethnic groups.

There is a clear age focus: in 40% of patients alopecia areata first appears before age 20, and in about 85% it manifests by age 40. Peak frequency is in the second and third decades of life. According to statistics, only about 6% of patients are older than 60.

There is also familial clustering in 10–25% of cases, pointing to a genetic component. About one third of patients are atopic—they have increased allergy readiness.

Typical Course Forms: Areata, Totalis, Universalis

Alopecia areata shows a broad clinical picture with various course forms:

• Circumscribed alopecia areata (alopecia areata circumscripta): One or more well-defined hairless areas on the scalp. This is the most common form, about 65% of cases.
• Alopecia totalis: Complete hair loss on the scalp. This form affects about 7% of patients.
• Alopecia universalis: Complete hair loss on the head, face, and body, affecting about 3% of patients.
• Ophiasis-type alopecia areata: Band-like hair loss at the nape and temples—a less favorable course form.
• Inverse ophiasis (sisaipho): Hair loss on the crown while lateral and posterior scalp areas are spared.
• Diffuse alopecia areata: Diffuse loss and density reduction without typical round patches.

In most people only a few bald spots develop, which often regrow spontaneously even without treatment. The course can be wave-like, however, with recurring flares over years.

How the Immune System Attacks Hair Follicles

Alopecia areata is a T-cell-mediated immune reaction directed specifically against the deep portion of the hair root. This misguided immune response leads to destruction of follicles, so hairs fall out. Hair-forming potential remains, however, because stem cells are not attacked.

Loss of Immune Privilege in the Hair Follicle

The hair follicle is one of the few organs in the body protected by special immune privilege. This privilege mainly rests on two mechanisms: first, MHC class I molecules and beta-2-microglobulin are downregulated, preventing presentation of autoantigens to T cells. Second, the follicle produces several immunosuppressive substances such as TGF-β1, IL-10, melanocyte-stimulating hormone (αMSH), and vasoactive intestinal peptide (VIP).

In alopecia areata this protective mechanism breaks down. MHC-I expression is upregulated, enabling recognition by antigen-presenting cells. Loss of immune privilege is a fundamental prerequisite for disease development. Without this protection, T cells can recognize and attack the follicle.

Role of T Cells and Cytokines (e.g. Interferon-γ)

At the center of pathogenesis are CD8+ T cells with NKG2D receptors. These play a decisive role in attacking follicles. Animal-model experiments clearly show that both NKG2D and CD8 must be expressed on T cells for alopecia areata to develop.

Especially important is interferon-gamma (IFN-γ) produced by T cells, identified as the driving factor for inhibiting immune privilege. Animal experiments confirm this central role: when IFN-γ is blocked in C3H/HeJ mice, they no longer develop alopecia areata and become resistant to the disease.

Further cytokines are involved in pathogenesis:

• Interferon-γ, interleukin-2, and interleukin-15 have been identified as decisive cytokines in alopecia areata
• Elevated levels of IL-17A, IL-17F, IL-21, and IL-23 point to a type-17 immune response
• Type-2 cytokines such as IL-33, IL-31, and IL-17E/25 are also significantly elevated in alopecia areata

This complex cytokine signature explains why the disease was originally considered a type-1 inflammatory disease, while today it is known that Th2 and Th17 immune responses also play a role.

Lymphocytic Infiltrate Around the Hair Bulb

Histologically, alopecia areata shows a characteristic dense peri- and intrafollicular inflammatory-cell infiltrate described as a "swarm of bees." This infiltrate concentrates on the bulb region of the follicle, while the stem-cell niche in the permanent part of the follicle is spared.

The infiltrate includes CD8+ and CD4+ T cells, mast cells, natural killer (NK) cells, and dendritic cells. The time course is notable: CD8+ T cells are usually the first to enter intrafollicular areas, followed by dendritic cells and macrophages. CD4+ T cells are found later in the disease course, when follicle integrity is already severely disturbed.

Clinically this progressive destruction manifests as so-called "exclamation-mark hairs"—hairs that become thinner toward the scalp. Important to understand: the immune reaction remains limited to the deep part of the follicle; the epidermal part is preserved. That explains why alopecia areata does not scar and why the potential for hair formation is not lost even after long disease duration.

Genetic and Familial Risk Factors

Genetic factors play a significant role in the development of alopecia areata. It is a multifactorial disease in which genetics and environmental influences interact.

Heritability and Familial Clustering

Familial clustering of alopecia areata is a clear hint at the genetic component. Studies show that in about 15% to 30% of patients at least one other family member is also affected. Expression varies between family members, however.

For first-degree relatives of someone affected, disease risk is clearly above the population average. Siblings have a risk of about 7%, parents almost 8%, and children just under 6%. For second-degree relatives the risk of 1–2% matches the general population. These figures show: the genetic component is important but not alone decisive for disease onset.

Involved Genes and HLA Associations

Molecular-genetic studies support the hypothesis of an autoimmune origin of alopecia areata. Especially notable is the HLA region (human leukocyte antigen region) on the short arm of chromosome 6. Various HLA markers show association with the disease, including DR-4, DR-5, DR-6, DR-7, DR-11, DQ3, and DQB-1.

Besides the HLA region, the following genes have been linked to alopecia areata:

• TRAF1/C5 (tumor necrosis factor receptor-associated factor 1)
• PTPN22 (protein tyrosine phosphatase non-receptor type 22)
• CTLA4 (cytotoxic T lymphocyte-associated antigen 4)
• Interleukin genes such as IL-2/IL-21, IL-2RA, and IL-13

There is also a link with Down syndrome (trisomy 21), in which alopecia areata occurs in 10% of cases—above average.

Although numerous genetic associations are known, there is so far no routine diagnostics for the genetically complex forms of alopecia areata. Further research is needed to identify individual mutations that can be tested for in a targeted way.

Differences in Children and Adults

Genetic predisposition shows differences between age groups. First manifestation in childhood before puberty is considered a prognostically unfavorable factor. Early manifestation is often linked to a more severe and prolonged course.

Interestingly, prevalence is somewhat higher in children than in adults. Overall peak manifestation is still in the second and third decades of life, suggesting that genetic factors combined with age-specific environmental influences shape disease risk.

For children with family history, the risk of developing alopecia areata is clearly elevated. Genetic predisposition does not inevitably mean the disease will break out—a hint at the complex interaction between genes and environment.

External Triggers and Environmental Factors

While genetic factors form the basis for alopecia areata, various external triggers can spark actual disease onset. These environmental factors play a decisive role in destabilizing the immune system and reactivating disease.

Psychological Stress as a Trigger

The link between hair loss and psychological stress is well documented scientifically. With ongoing stress the body releases more stress hormones such as cortisol and adrenaline, which can impair follicle growth and disrupt the natural hair cycle.

Emotional and physical stress can throw the immune system out of balance so follicles react abnormally. This is not ordinary everyday stress but severe chronic stress situations. Research shows that lasting stress can even worsen and accelerate already existing patchy hair loss.

The good news: once the stress or burdening situation is overcome, hair often grows back—sometimes even stronger and more pigmented than before.

Infections and Hormonal Changes

Besides stress, the following factors can trigger onset of alopecia areata:

• Infections: Viral infections and other infectious diseases can cause fluctuations in immune activity and thereby trigger a flare
• Hormonal changes: Especially in women, pregnancy, postpartum period, menstrual irregularities, and menopause can pave the way for alopecia areata

Allergies, local scalp injuries, and certain medications can also act as triggers. In men, conditions linked to testosterone levels occasionally occur.

Ruling Out Malnutrition and Pollutants

Although malnutrition is a less common cause of hair loss, it can still play a role:

• Iron deficiency can lead to telogen effluvium, especially in women. Hair growth normalizes only when serum ferritin is above 70 ng/ml
• Biotin deficiency can also lead to diffuse hair loss; many patients benefit from biotin therapy
• Zinc deficiency presents with eczematous skin changes, nail dystrophies, and brittle hair
• Vitamin D deficiency negatively affects hair health

Environmental pollutants also play a growing role: certain pollutants such as fine particulate matter can trigger inflammatory reactions and have been linked to alopecia areata. Other environmental toxins such as tobacco smoke and phthalates have endocrine effects, while pesticides and heavy metals are associated with both alopecia areata and acute anagen effluvium.

Important: these external factors count as triggers, not as true causes. In genetically predisposed people, however, they can provide the decisive push for disease onset.

Diagnostics and Course of the Disease

Clinical examination is central to diagnosing alopecia areata. Typically, round hairless patches appear with short broken hairs at the edge that resemble exclamation marks.

Hair-Pull Test and Dermatoscopy

The hair-pull test is a valuable tool for assessing disease activity. About 20–50 hairs at the edge of bald patches are gently but firmly pulled away from the scalp. If more than 10% of the grasped hairs remain between the fingers, that points to active shedding. For valid results, hair should not have been washed for at least one day.

Dermatoscopy (trichoscopy) enables precise examination of follicles with a special hand microscope. Characteristic findings include "black dots," "yellow dots," and short broken hairs. This examination is sufficient for diagnosis in most cases.

SALT Score for Severity Assessment

The Severity of Alopecia Tool (SALT score) provides objective assessment of severity. The scalp is divided into four quadrants and the percentage of hair loss in each area is determined. The sum yields the total score, where 100 means complete hair loss. Based on the SALT score one distinguishes:

• Mild/Limited (1–20)
• Moderate (21–49)
• Severe (50–94)
• Very Severe (95–100)

Spontaneous Healing vs. Chronic Course

The course of alopecia areata is unpredictable. In more than 50% of patients, hair grows back spontaneously within six to twelve months. Prognosis worsens with extensive involvement, onset before puberty, and involvement of temporal and occipital scalp (ophiasis type).

About 30–50% of people who initially heal also experience a relapse. Regrown hairs are thin and unpigmented at first, then later take on the appearance of normal terminal hairs again.

## Conclusion

Alopecia areata is undoubtedly a complex autoimmune disease arising from the interplay of genetic factors and environmental influences. The misguided immune system attacks healthy follicles; loss of immune privilege and the T-cell-mediated reaction are the central mechanisms. Especially notable remains the fact that the hair root stays intact in this disease, so recovery is possible even after years.

Familial clustering underscores the genetic component, while external factors such as severe stress, infections, or hormonal changes can act as triggers. Diagnosis is primarily clinical through the characteristic appearance of round bald patches, plus hair-pull test and dermatoscopy.

The course varies individually. Encouragingly, more than half of those affected experience spontaneous improvement within a year. Many patients must still reckon with relapses or chronic courses, especially with early onset or certain forms such as the ophiasis type.

Hope comes from advancing research into immunomodulating therapies that offer promising approaches. Deeper understanding of the underlying immunological mechanisms paves the way for targeted treatment strategies. Those affected should know they are not alone with this disease and should seek medical support for the best possible treatment outcomes.