Alopecia Areata

🏷

Classifications

Harrison's Chapter

⚙

Pathophysiology

4

Loss of Hair Follicle Immune Privilege

Hair follicles normally maintain immune privilege through low MHC class I expression and local immunosuppressive factors. Collapse of immune privilege exposes hair follicle antigens to immune surveillance.

Immune Response link

Show evidence (2 references)

PMID:32682334 SUPPORT

"Anagen stage hair follicles (HFs) exhibit "immune privilege (IP)" from the level of the bulge downwards to the bulb. Both passive and active IP mechanisms protect HFs from physiologically undesired immune responses and limit immune surveillance. IP is relative, not absolute, and is primarily..."

This review describes the mechanisms of hair follicle immune privilege including reduced MHC expression and immunosuppressive signaling, confirming the normal maintenance of immune privilege in anagen hair follicles.

PMID:38673994 SUPPORT

"Immune privilege collapse has been extensively investigated in AA pathogenesis, including the suppression of immunomodulatory factors (e.g., transforming growth factor-β (TGF-β), programmed death-ligand 1 (PDL1), interleukin-10 (IL-10), α-melanocyte-stimulating hormone (α-MSH), and macrophage..."

This study documents the specific immunomodulatory factors that are suppressed during immune privilege collapse in AA, along with enhanced MHC expression that enables autoreactive T cell recognition.

CD8+ T Cell-Mediated Attack

NKG2D-expressing CD8+ T cells infiltrate the hair bulb and attack hair follicle cells. IFN-gamma produced by these cells induces MHC class I upregulation and further T cell recruitment.

CD8+ T Cell link

T Cell Cytotoxicity link

Show evidence (2 references)

PMID:32682334 SUPPORT

"In AA, lesional HFs are rapidly infiltrated by NKG2D + T cells and natural killer (NK) cells, while perifollicular mast cells acquire a profoundly pro-inflammatory phenotype and interact with autoreactive CD8+ T cells. Using animal models, significant functional evidence has accumulated that..."

This demonstrates that CD8+ T cells alone are sufficient to induce alopecia areata in animal models, establishing their central role as disease drivers. The NKG2D+ phenotype and IFN-gamma production are key features.

PMID:37365384 SUPPORT

"Pathogenic activity of CD44s-hiCD49dlo CD8+ T cells was mediated by NKG2D-dependent innate-like cytotoxicity, which was further augmented by IL-15 stimulation and triggered disease onset."

This identifies the specific mechanism of CD8+ T cell cytotoxicity in AA through NKG2D-dependent pathways, and demonstrates that IL-15 stimulation enhances their pathogenic activity.

JAK-STAT Signaling Activation

IFN-gamma signaling through JAK1/2-STAT1 pathway drives disease pathogenesis. This pathway is a key therapeutic target, with JAK inhibitors showing efficacy in alopecia areata.

JAK-STAT Signaling link

Show evidence (3 references)

PMID:38673994 SUPPORT

"Both AA and vitiligo are autoimmune diseases that share commonalities in pathogenesis, including the involvement of plasmacytoid dendritic cells (and interferon-α (IFN- α) signaling pathways) and cytotoxic CD8+ T lymphocytes (and activated IFN-γ signaling pathways)."

This establishes that IFN-gamma signaling pathways are activated in AA pathogenesis, providing the mechanistic basis for JAK-STAT pathway involvement since these cytokines signal through JAK-STAT.

PMID:37326792 SUPPORT

"In two pivotal placebo-controlled phase 3 clinical trials in adults with severe alopecia areata, oral baricitinib once daily was associated with clinically meaningful scalp, eyebrow, and eyelash hair regrowth over 36 weeks."

The clinical efficacy of baricitinib, a JAK1/2 inhibitor, in promoting hair regrowth provides direct evidence that the JAK-STAT pathway is a critical therapeutic target in AA pathogenesis.

PMID:38338982 SUPPORT

"Interferon-γ (IFNγ) treatment increased the secretion of CXCL12 from DFs through the signal transducer and activator of transcription 3 (STAT3) pathway, and αCXCL12 treatment protected the hair follicle from IFNγ in hair organ culture."

This demonstrates that IFN-gamma signals through STAT3 to drive pathogenic chemokine production, providing direct mechanistic evidence for JAK-STAT pathway activation in AA.

Chemokine-Mediated Immune Cell Recruitment

CXCL12 and other chemokines recruit and activate CD8+ T cells to the hair follicle. IFN-gamma induces CXCL12 production from dermal fibroblasts, amplifying immune cell infiltration and contributing to disease progression.

Immune Response link

Show evidence (2 references)

PMID:38338982 SUPPORT

"CXCL12 expression is increased in AA mice, while s.c. injection of αCXCL12 significantly inhibited hair loss in AA mice and reduced the number of CD8+, MHC-I+, and MHC-II+ cells in the skin. In addition, injection of αCXCL12 also prevented the onset of AA and reduced the number of CD8+ cells."

This demonstrates that CXCL12 drives CD8+ T cell recruitment to hair follicles, and blocking this chemokine can prevent disease onset and reduce immune cell infiltration in AA.

PMID:38673994 PARTIAL

"Blood chemokine C-X-C motif ligand 9 (CXCL9) and CXCL10 are elevated in both diseases."

Elevated CXCL9 and CXCL10 chemokines in AA blood support the role of chemokine-mediated immune cell recruitment in disease pathogenesis.

●

Phenotypes

2

Patchy Hair Loss VERY_FREQUENT Dermatological HP:0001596

Well-circumscribed patches, exclamation mark hairs at edges

Nail Changes OCCASIONAL Dermatological HP:0001597

Pitting, trachyonychia

Show evidence (1 reference)

PMID:32682334 NO_EVIDENCE

"Collapse of anagen hair bulb IP is an essential prerequisite for the development of alopecia areata (AA). In AA, lesional HFs are rapidly infiltrated by NKG2D + T cells and natural killer (NK) cells, while perifollicular mast cells acquire a profoundly pro-inflammatory phenotype and interact..."

While this reference focuses on hair follicle pathology, the immune mechanisms described also affect nail matrix, explaining the nail changes seen in AA patients.

🧬

Genetic Associations

4

HLA-DRB1 (Risk Factor)

CTLA4 (Risk Factor)

IL2RA (Risk Factor)

PTPN22 (Risk Factor)

💊

Treatments

4

Topical Corticosteroids

First-line for limited disease.

Intralesional Corticosteroids

For localized patches.

JAK Inhibitors

Baricitinib FDA-approved for severe alopecia areata.

Topical Immunotherapy

DPCP or SADBE for extensive disease.

🌍

Environmental Factors

2

Psychological Stress

May trigger or exacerbate disease

Viral Infections

Possible trigger

🔬

Biochemical Markers

1

Anti-Hair Follicle Antibodies (Variable)

Context: Not routinely tested

{ }

Source YAML

click to show

name: Alopecia Areata
creation_date: '2025-12-19T01:12:52Z'
updated_date: '2026-02-17T21:53:14Z'
category: Autoimmune
parents:
- Autoimmune Disease
- Skin Disease
disease_term:
  preferred_term: Alopecia Areata
  term:
    id: MONDO:0005340
    label: alopecia areata
description: >-
  An autoimmune disease targeting hair follicles, resulting in non-scarring
  hair loss. Ranges from patchy hair loss to complete scalp (alopecia totalis)
  or body hair loss (alopecia universalis). Hair follicles retain regenerative
  capacity, allowing potential regrowth.
pathophysiology:
- name: Loss of Hair Follicle Immune Privilege
  description: >-
    Hair follicles normally maintain immune privilege through low MHC class I
    expression and local immunosuppressive factors. Collapse of immune privilege
    exposes hair follicle antigens to immune surveillance.
  biological_processes:
  - preferred_term: Immune Response
    term:
      id: GO:0006955
      label: immune response
  evidence:
  - reference: PMID:32682334
    reference_title: "Hair follicle immune privilege and its collapse in alopecia areata."
    supports: SUPPORT
    snippet: >-
      Anagen stage hair follicles (HFs) exhibit "immune privilege (IP)" from the level
      of the bulge downwards to the bulb. Both passive and active IP mechanisms
      protect HFs from physiologically undesired immune responses and limit immune
      surveillance. IP is relative, not absolute, and is primarily based on absent,
      or
      greatly reduced, intra-follicular antigen presentation via MHC class I and II
      molecules, along with prominent expression of "no danger" signals like CD200
      and
      the creation of an immunoinhibitory signalling milieu generated by the secretory
      activities of HFs.
    explanation: >-
      This review describes the mechanisms of hair follicle immune privilege including
      reduced MHC expression and immunosuppressive signaling, confirming the normal
      maintenance of immune privilege in anagen hair follicles.
  - reference: PMID:38673994
    reference_title: "Pathogenesis of Alopecia Areata and Vitiligo: Commonalities and Differences."
    supports: SUPPORT
    snippet: >-
      Immune privilege collapse has been extensively investigated in AA pathogenesis,
      including the suppression of immunomodulatory factors (e.g., transforming growth
      factor-β (TGF-β), programmed death-ligand 1 (PDL1), interleukin-10 (IL-10),
      α-melanocyte-stimulating hormone (α-MSH), and macrophage migration inhibitory
      factor (MIF)) and enhanced expression of the major histocompatibility complex
      (MHC) throughout hair follicles.
    explanation: >-
      This study documents the specific immunomodulatory factors that are suppressed
      during immune privilege collapse in AA, along with enhanced MHC expression that
      enables autoreactive T cell recognition.
- name: CD8+ T Cell-Mediated Attack
  description: >-
    NKG2D-expressing CD8+ T cells infiltrate the hair bulb and attack
    hair follicle cells. IFN-gamma produced by these cells induces MHC class I
    upregulation and further T cell recruitment.
  cell_types:
  - preferred_term: CD8+ T Cell
    term:
      id: CL:0000625
      label: CD8-positive, alpha-beta T cell
  biological_processes:
  - preferred_term: T Cell Cytotoxicity
    term:
      id: GO:0001913
      label: T cell mediated cytotoxicity
  evidence:
  - reference: PMID:32682334
    reference_title: "Hair follicle immune privilege and its collapse in alopecia areata."
    supports: SUPPORT
    snippet: >-
      In AA, lesional HFs are rapidly infiltrated by NKG2D + T cells and natural
      killer (NK) cells, while perifollicular mast cells acquire a profoundly
      pro-inflammatory phenotype and interact with autoreactive CD8+ T cells. Using
      animal models, significant functional evidence has accumulated that demonstrates
      the dominance of the immune system in AA pathogenesis. Purified CD8+T-cell and
      NK cell populations alone, which secrete fγ, suffice to induce the AA phenotype,
      while CD4+T-cells aggravate it, and Tregs and iNKT cells may provide relative
      protection against AA development.
    explanation: >-
      This demonstrates that CD8+ T cells alone are sufficient to induce alopecia
      areata in animal models, establishing their central role as disease drivers.
      The NKG2D+ phenotype and IFN-gamma production are key features.
  - reference: PMID:37365384
    reference_title: "A virtual memory CD8(+) T cell-originated subset causes alopecia areata through innate-like cytotoxicity."
    supports: SUPPORT
    snippet: >-
      Pathogenic activity of CD44s-hiCD49dlo CD8+ T cells was mediated by NKG2D-dependent
      innate-like cytotoxicity, which was further augmented by IL-15 stimulation and
      triggered disease onset.
    explanation: >-
      This identifies the specific mechanism of CD8+ T cell cytotoxicity in AA through
      NKG2D-dependent pathways, and demonstrates that IL-15 stimulation enhances their
      pathogenic activity.
- name: JAK-STAT Signaling Activation
  description: >-
    IFN-gamma signaling through JAK1/2-STAT1 pathway drives disease
    pathogenesis. This pathway is a key therapeutic target, with JAK
    inhibitors showing efficacy in alopecia areata.
  biological_processes:
  - preferred_term: JAK-STAT Signaling
    term:
      id: GO:0007259
      label: cell surface receptor signaling pathway via JAK-STAT
  evidence:
  - reference: PMID:38673994
    reference_title: "Pathogenesis of Alopecia Areata and Vitiligo: Commonalities and Differences."
    supports: SUPPORT
    snippet: >-
      Both AA and vitiligo are autoimmune diseases that share commonalities in
      pathogenesis, including the involvement of plasmacytoid dendritic cells
      (and interferon-α (IFN- α) signaling pathways) and cytotoxic CD8+ T
      lymphocytes (and activated IFN-γ signaling pathways).
    explanation: >-
      This establishes that IFN-gamma signaling pathways are activated in AA
      pathogenesis, providing the mechanistic basis for JAK-STAT pathway involvement
      since these cytokines signal through JAK-STAT.
  - reference: PMID:37326792
    reference_title: "Baricitinib: A Review in Severe Alopecia Areata."
    supports: SUPPORT
    snippet: >-
      In two pivotal placebo-controlled phase 3 clinical trials in adults
      with severe alopecia areata, oral baricitinib once daily was associated with
      clinically meaningful scalp, eyebrow, and eyelash hair regrowth over 36 weeks.
    explanation: >-
      The clinical efficacy of baricitinib, a JAK1/2 inhibitor, in promoting hair
      regrowth provides direct evidence that the JAK-STAT pathway is a critical
      therapeutic target in AA pathogenesis.
  - reference: PMID:38338982
    reference_title: "CXCL12 Neutralizing Antibody Promotes Hair Growth in Androgenic Alopecia and Alopecia Areata."
    supports: SUPPORT
    snippet: >-
      Interferon-γ (IFNγ) treatment increased the secretion of CXCL12 from DFs through
      the signal transducer and activator of transcription 3 (STAT3) pathway, and
      αCXCL12 treatment protected the hair follicle from IFNγ in hair organ culture.
    explanation: >-
      This demonstrates that IFN-gamma signals through STAT3 to drive pathogenic
      chemokine production, providing direct mechanistic evidence for JAK-STAT
      pathway activation in AA.
- name: Chemokine-Mediated Immune Cell Recruitment
  description: >-
    CXCL12 and other chemokines recruit and activate CD8+ T cells to the hair
    follicle. IFN-gamma induces CXCL12 production from dermal fibroblasts,
    amplifying immune cell infiltration and contributing to disease progression.
  biological_processes:
  - preferred_term: Immune Response
    term:
      id: GO:0006955
      label: immune response
  evidence:
  - reference: PMID:38338982
    reference_title: "CXCL12 Neutralizing Antibody Promotes Hair Growth in Androgenic Alopecia and Alopecia Areata."
    supports: SUPPORT
    snippet: >-
      CXCL12 expression is increased in AA mice, while s.c. injection of
      αCXCL12 significantly inhibited hair loss in AA mice and reduced the number
      of
      CD8+, MHC-I+, and MHC-II+ cells in the skin. In addition, injection of αCXCL12
      also prevented the onset of AA and reduced the number of CD8+ cells.
    explanation: >-
      This demonstrates that CXCL12 drives CD8+ T cell recruitment to hair follicles,
      and blocking this chemokine can prevent disease onset and reduce immune cell
      infiltration in AA.
  - reference: PMID:38673994
    reference_title: "Pathogenesis of Alopecia Areata and Vitiligo: Commonalities and Differences."
    supports: PARTIAL
    snippet: >-
      Blood chemokine C-X-C motif ligand 9 (CXCL9) and CXCL10 are elevated in both
      diseases.
    explanation: >-
      Elevated CXCL9 and CXCL10 chemokines in AA blood support the role of
      chemokine-mediated immune cell recruitment in disease pathogenesis.
phenotypes:
- name: Patchy Hair Loss
  category: Dermatological
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Alopecia
    term:
      id: HP:0001596
      label: Alopecia
  notes: Well-circumscribed patches, exclamation mark hairs at edges
- name: Nail Changes
  category: Dermatological
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Abnormal Nail Morphology
    term:
      id: HP:0001597
      label: Abnormal nail morphology
  notes: Pitting, trachyonychia
  evidence:
  - reference: PMID:32682334
    reference_title: "Hair follicle immune privilege and its collapse in alopecia areata."
    supports: NO_EVIDENCE
    snippet: >-
      Collapse of anagen hair bulb IP is an essential prerequisite for the development
      of alopecia areata (AA). In AA, lesional HFs are rapidly infiltrated by NKG2D
      + T
      cells and natural killer (NK) cells, while perifollicular mast cells acquire
      a
      profoundly pro-inflammatory phenotype and interact with autoreactive CD8+ T
      cells.
    explanation: >-
      While this reference focuses on hair follicle pathology, the immune mechanisms
      described also affect nail matrix, explaining the nail changes seen in AA patients.
biochemical:
- name: Anti-Hair Follicle Antibodies
  presence: Variable
  context: Not routinely tested
genetic:
- name: HLA-DRB1
  association: Risk Factor
- name: CTLA4
  association: Risk Factor
- name: IL2RA
  association: Risk Factor
- name: PTPN22
  association: Risk Factor
environmental:
- name: Psychological Stress
  notes: May trigger or exacerbate disease
- name: Viral Infections
  notes: Possible trigger
treatments:
- name: Topical Corticosteroids
  description: First-line for limited disease.
- name: Intralesional Corticosteroids
  description: For localized patches.
- name: JAK Inhibitors
  description: Baricitinib FDA-approved for severe alopecia areata.
- name: Topical Immunotherapy
  description: DPCP or SADBE for extensive disease.
classifications:
  harrisons_chapter:
  - classification_value: skin disorder
  - classification_value: autoimmune disease
references:
- reference: DOI:10.1073/pnas.2305764120
  title: Functional interrogation of lymphocyte subsets in alopecia areata using
    single-cell RNA sequencing
  findings: []
- reference: DOI:10.3389/fimmu.2024.1444777
  title: 'Humanized CXCL12 antibody delays onset and modulates immune response in
    alopecia areata mice: insights from single-cell RNA sequencing'
  findings: []
- reference: DOI:10.3389/fimmu.2025.1621492
  title: 'From mechanisms to therapies: current advances breakthroughs in alopecia
    areata immunopathology'
  findings: []
- reference: DOI:10.3389/fimmu.2025.1681163
  title: 'Alopecia areata: from immunopathogenesis to emerging therapeutic approaches'
  findings: []
- reference: DOI:10.3390/ijms25084409
  title: 'Pathogenesis of Alopecia Areata and Vitiligo: Commonalities and Differences'
  findings: []
- reference: DOI:10.3390/ijms25115652
  title: Deciphering the Complex Immunopathogenesis of Alopecia Areata
  findings: []
- reference: DOI:10.53582/amj255170s
  title: 'DISRUPTION OF HAIR FOLLICLE IMMUNE PRIVILEGE IN ALOPECIA AREATA: ENIGMATIC
    MECHANISMS AND EMERGING CONCEPTS'
  findings: []

📚

References & Deep Research