Autoimmune Skin Diseases

Mechanistic groupings

• Loss of B-cell tolerance to structural proteins: pemphigus (desmoglein 1/3) and pemphigoid (BP180/BP230) arise when autoreactive B cells escape tolerance and produce pathogenic IgG; disease activity tracks with antibody titer, which is why B-cell depletion works.
• Molecular mimicry / cross-reactive antigen: dermatitis herpetiformis reflects IgA against epidermal transglutaminase generated in response to gluten-driven tissue transglutaminase immunity in celiac disease — a gut-skin axis mechanism.
• Immune complex and interface injury: SLE and subacute cutaneous lupus involve UV-induced keratinocyte apoptosis exposing Ro/SSA, type I interferon amplification, and immune-complex deposition.
• Fibrotic/vasculopathic: systemic sclerosis begins with endothelial injury and TGF-β–driven myofibroblast activation rather than blistering.
• Cytotoxic T-cell attack on a cell lineage: vitiligo (CD8+ T cells against melanocytes, IFN-γ/CXCL10 axis) and alopecia areata (collapse of hair-follicle immune privilege) — the rationale for JAK inhibition.
• Paraneoplastic: paraneoplastic pemphigus with Castleman disease, non-Hodgkin lymphoma, or CLL; dermatomyositis with ovarian, lung, colorectal, and nasopharyngeal carcinoma (anti-TIF1-γ).

Non-modifiable risk factors

• Genetics/HLA: HLA-DRB1*04:02 and DQB1*05:03 in pemphigus vulgaris (enriched in Ashkenazi Jewish patients); HLA-DQ2/DQ8 in dermatitis herpetiformis; early complement deficiency (C1q, C4) strongly predisposes to SLE.
• Age and sex: bullous pemphigoid clusters in the elderly and correlates with neurodegenerative disease; SLE, scleroderma, and dermatomyositis are markedly female-predominant in reproductive years.
• Family history/personal autoimmunity: vitiligo clusters with autoimmune thyroid disease, type 1 diabetes, and pernicious anemia.

Modifiable/exposure-related

• Ultraviolet light: photoprovokes lupus and dermatomyositis rashes; sun protection is formal therapy in ACR/EULAR-informed lupus care.
• Drugs: DPP-4 inhibitors (gliptins) and PD-1/PD-L1 checkpoint inhibitors → bullous pemphigoid; thiol drugs (penicillamine, captopril) → pemphigus; hydrochlorothiazide and terbinafine → subacute cutaneous lupus; hydralazine, procainamide, isoniazid, minocycline, TNF inhibitors → drug-induced lupus (anti-histone antibodies).
• Gluten ingestion: obligate driver of dermatitis herpetiformis.
• Smoking: associated with more severe and more extensive cutaneous lupus and with worse digital ischemia in scleroderma; observational data have also suggested reduced antimalarial responsiveness in smokers, though findings across studies are inconsistent.
• Occupational exposures: silica, organic solvents, and vinyl chloride are linked to scleroderma-like disease.

Autoimmune Blistering Diseases

• Pemphigus vulgaris: IgG autoantibodies against desmoglein 3 (mucosal involvement) and desmoglein 1 (mucocutaneous involvement) → disruption of desmosomes → acantholysis (loss of cell-to-cell adhesion) → intraepidermal bulla formation; mediated by IgG1 and IgG3 subtypes; antibodies directly induce keratinocyte detachment
• Bullous pemphigoid: IgG and IgM autoantibodies against BP180 and BP230 (hemidesmosomal antigens at the dermal-epidermal junction) → complement activation (especially C3) → recruitment of neutrophils and eosinophils → subepidermal blister formation; predominantly TH2-mediated response
• Dermatitis herpetiformis: IgA deposition along the dermal-epidermal junction associated with celiac disease (tissue transglutaminase cross-reactivity) → neutrophil recruitment → papillary dermal microabscesses

Systemic Autoimmune Skin Diseases

• SLE: Type III hypersensitivity (immune complex deposition) + type II hypersensitivity (anti-dsDNA, anti-nucleosome antibodies) → complement activation (C1q, C3, C4 consumption) → basement membrane thickening ("wire-loop lesions") → interface dermatitis with liquefactive degeneration of basal layer
• Scleroderma: Aberrant TGF-β signaling and PDGF activation → excessive collagen synthesis and deposition by myofibroblasts → loss of immune tolerance with anti-Scl-70 and anti-centromere antibodies; endothelial injury and microvascular disease are early pathogenic events
• Pemphigoid gestationis: Pregnancy-induced IgG autoantibodies against BP180 → primarily complement-mediated (C3 predominance) → subepidermal blistering; often improves postpartum due to hormonal triggers

Pemphigus Vulgaris

• Painful mucosal erosions as initial presentation (>90% of cases) — mouth, pharynx, conjunctiva; erosions are raw and non-healing
• Flaccid bullae on erythematous base that rupture easily, leaving erosions; bullae lack firm walls
• Positive Nikolsky sign (gentle rubbing of skin produces sloughing) — hallmark finding indicating acantholysis
• Cutaneous lesions typically appear weeks to months after mucosal involvement; commonly affects face, scalp, intertriginous areas, and lower abdomen

Bullous Pemphigoid

• Tense, fluid-filled bullae on normal or erythematous skin; firm bullae that do NOT rupture easily (unlike pemphigus)
• Negative Nikolsky sign (indicates subepidermal pathology preserving epidermis)
• Urticarial or eczematous plaques with grouped vesicles; intense pruritus often precedes bulla formation
• Flexural distribution common (lower abdomen, inner thighs, axillae); oral involvement is rare or absent
• Predominantly affects elderly patients (peak age 60-80 years)

Dermatitis Herpetiformis

• Intensely pruritic grouped papules and vesicles with characteristic "urticaria-like" appearance; the pruritus often precedes visible lesions
• Distribution on extensor surfaces (elbows, knees, buttocks, shoulders) and scalp
• Excoriations are prominent from scratching; secondary infection common
• Always associated with celiac disease (even if asymptomatic) — patients have villous atrophy
• Responsive to dapsone dramatically (typically improves within 24-48 hours)

Systemic Lupus Erythematosus (Cutaneous Manifestations)

• Malar "butterfly" rash — classic presentation; fixed erythema over cheeks and bridge of nose, sparing the nasolabial folds
• Discoid lesions — well-demarcated erythematous plaques with follicular plugging and scarring; typically on sun-exposed areas
• Photosensitivity — exacerbation of skin lesions with UV exposure (diagnostic criterion)
• Oral ulcers, alopecia, Raynaud's phenomenon, and photosensitive rashes on dorsal hands
• May present with other systemic features: arthritis, nephritis, serositis, ANA positivity

Systemic Sclerosis (Scleroderma)

• Raynaud's phenomenon — often precedes skin changes by years; triphasic color changes (white→blue→red) in response to cold/stress
• Skin induration and thickening beginning distally (fingers) and progressing proximally in limited cutaneous disease; or widespread in diffuse disease
• Puffy fingers early on, followed by "waxy" skin with loss of normal skin folds and hair
• Microstomia (tight, restricted mouth opening) and facial mask appearance from facial skin tightening
• Sclerodactyly (skin tightening over fingers) and digital ulcers on fingertips
• Diffuse vs. limited subtypes: limited (CREST syndrome) has better prognosis; diffuse has rapid progression and visceral involvement

Pemphigoid Gestationis

• Typically emerges in 2nd or 3rd trimester; may flare postpartum
• Intensely pruritic urticarial papules evolving to vesicles and bullae; periumbilical distribution common
• Erythematous plaques with vesicles similar to bullous pemphigoid
• Fetal involvement rare; usually self-limited but may recur in future pregnancies or with oral contraceptives

Serologic and Histopathologic Approach

• Direct Immunofluorescence (DIF) of perilesional skin or oral mucosa — GOLD STANDARD for diagnosing autoimmune bullous diseases:
• Pemphigus vulgaris: IgG and C3 in intercellular pattern (throughout epidermis) on perilesional skin
• Bullous pemphigoid: IgG and C3 in linear pattern along basement membrane zone on perilesional or normal-appearing skin
• Dermatitis herpetiformis: Granular IgA at dermal-epidermal junction
• SLE: IgG, IgM, IgA, and C3 at basement membrane ("full house" pattern) in lupus band test

• Indirect Immunofluorescence (IIF) using patient serum on normal skin substrate:
• Pemphigus: IgG against desmoglein (can be quantified; higher titers correlate with disease activity)
• Bullous pemphigoid: IgG and/or IgM against basement membrane zone; less sensitive than DIF

• Serum Antibody Testing:
• Pemphigus vulgaris: Anti-desmoglein 3 ELISA (and anti-desmoglein 1) — specific and correlates with disease severity
• Bullous pemphigoid: **Anti-BP180 and anti-BP230

Immediate stabilization (extensive erosive disease)

• Wound and fluid management: widespread pemphigus behaves like a burn — insensible losses, hypoalbuminemia, and Staphylococcus aureus or herpes simplex superinfection; culture, non-adherent dressings, and ophthalmology/ENT input for mucosal disease.
• Scleroderma renal crisis is a true emergency: abrupt severe hypertension with microangiopathic hemolysis and rising creatinine requires an ACE inhibitor — captopril is favored because its short half-life permits rapid titration (all ACE inhibitors, including captopril, are contraindicated in pregnancy).

First-line therapy

• Systemic corticosteroids: prednisone remains induction therapy for pemphigus vulgaris and for severe bullous pemphigoid when topical therapy is not feasible.
• Anti-CD20 monoclonal antibody: rituximab carries FDA approval for moderate-to-severe pemphigus vulgaris (approved 2018 on the basis of the Ritux 3 trial) and is endorsed as first-line, steroid-sparing therapy with corticosteroids by the international consensus/expert recommendations on pemphigus (Murrell et al.); screen for hepatitis B and tuberculosis before dosing.
• Superpotent topical corticosteroid: whole-body clobetasol propionate 0.05% is first-line for bullous pemphigoid in both localized and extensive disease — in the pivotal Joly trials it was at least as effective as oral prednisone, with better survival in extensive disease and less systemic toxicity; clobetasol is also first-line for discoid lupus lesions.
• Sulfone: dapsone produces dramatic response in dermatitis herpetiformis within 24–48 hours — check G6PD first.
• Antimalarial: hydroxychloroquine is the backbone for all cutaneous and systemic lupus per ACR guidance; dose ≤5 mg/kg actual body weight with baseline and annual retinal screening (American Academy of Ophthalmology).

Escalation/second-line

• Antimetabolites: mycophenolate mofetil, azathioprine (check TPMT/NUDT15), or methotrexate as steroid-sparing agents.
• Anti-inflammatory antibiotic: doxycycline for bullous pemphigoid in frail elderly patients.
• IVIG or plasma exchange for refractory or fulminant pemphigus; IVIG is FDA-approved for dermatomyositis.
• Targeted agents: topical ruxolitinib for nonsegmental vitiligo and oral JAK inhibitors (baricitinib) for severe alopecia areata; narrowband UVB phototherapy plus topical calcineurin inhibitors for vitiligo.
• Vasodilators: dihydropyridine calcium channel blockers (nifedipine) for Raynaud; PDE5 inhibitors or endothelin receptor antagonists for digital ulcers.

Definitive and contraindicated

• Lifelong gluten-free diet is definitive for dermatitis herpetiformis and permits dapsone withdrawal.
• Avoid high-dose glucocorticoids in diffuse systemic sclerosis — they precipitate renal crisis; avoid live vaccines and unscreened biologics during immunosuppression.

Disease-related

• Sepsis and fluid/electrolyte loss in pemphigus (emergency): barrier failure from acantholytic erosions; signaled by fever, purulent or malodorous erosions, hypotension, and hypoalbuminemia.
• Scleroderma renal crisis (emergency): intimal proliferation and renin surge cause malignant hypertension, schistocytes with thrombocytopenia, and acute kidney injury — highest risk in early diffuse disease and anti-RNA polymerase III positivity.
• Pulmonary arterial hypertension and interstitial lung disease in scleroderma: PAH classically complicates limited/CREST disease (anti-centromere), ILD complicates diffuse disease (anti-Scl-70); dyspnea with a loud P2 or a falling DLCO is the tip-off.
• Lupus nephritis (urgent): immune-complex glomerular injury; proteinuria, active urinary sediment, and falling C3/C4 with rising anti-dsDNA mandate biopsy per ACR guidance.
• Occult malignancy in dermatomyositis: highest with anti-TIF1-γ; age- and sex-appropriate cancer screening is required at diagnosis. Rapidly progressive ILD with anti-MDA5 amyopathic disease is life-threatening.
• Enteropathy-associated complications of dermatitis herpetiformis: untreated celiac disease causes iron and folate deficiency, osteoporosis, and small-bowel lymphoma.
• Scarring sequelae: discoid lupus causes permanent scarring alopecia and dyspigmentation; pemphigoid gestationis may cause transient neonatal blistering and prematurity/small-for-gestational-age infants.
• Psychosocial morbidity: vitiligo and alopecia areata carry high rates of depression and anxiety despite benign systemic course; vitiligo patches sunburn readily.

Treatment-related

• Glucocorticoids: hyperglycemia, osteoporosis with fragility fracture, avascular necrosis of the femoral head (new groin pain), adrenal suppression, and Pneumocystis infection at prolonged high dose.
• Rituximab: hypogammaglobulinemia, hepatitis B reactivation (screen before therapy), and rarely progressive multifocal leukoencephalopathy — new focal neurologic deficits demand MRI.
• Dapsone: dose-dependent hemolysis (severe in G6PD deficiency), methemoglobinemia (cyanosis with normal PaO₂, chocolate-brown blood), agranulocytosis, and dapsone hypersensitivity syndrome.
• Methylene blue is the antidote for symptomatic methemoglobinemia, but it is ineffective and potentially hemolytic in G6PD deficiency (NADPH-dependent reduction is impaired) — use exchange transfusion or high-dose ascorbic acid instead.
• Hydroxychloroquine: bull's-eye maculopathy, detected by screening before symptoms appear.
• Cyclophosphamide: hemorrhagic cystitis and bladder cancer, gonadal failure. Mycophenolate: teratogenicity. JAK inhibitors: boxed warnings for thrombosis, MACE, malignancy, and herpes zoster.

• Flaccid + oral erosions + positive Nikolsky = pemphigus vulgaris; tense + pruritic + no oral lesions + negative Nikolsky = bullous pemphigoid. The single best next step for either is perilesional (not lesional) skin biopsy for direct immunofluorescence — net-like/chicken-wire intercellular IgG in pemphigus versus linear IgG/C3 at the basement membrane in pemphigoid.
• Intensely pruritic grouped vesicles on extensor elbows and knees = dermatitis herpetiformis; check IgA tissue transglutaminase and start a gluten-free diet — the definitive therapy. Dapsone relieves itch within 48 hours but does nothing for the enteropathy; check G6PD before dapsone.
• New tense bullae in an elderly diabetic — look for a DPP-4 inhibitor (gliptin) or checkpoint inhibitor in the medication list; drug withdrawal is part of treatment. Whole-body superpotent topical clobetasol, not oral prednisone, is the preferred first-line therapy even when bullous pemphigoid is extensive.
• Malar rash spares the nasolabial folds; the dermatomyositis heliotrope rash and Gottron papules do not spare, and dermatomyositis involves the knuckles while lupus spares them. That knuckle-versus-interphalangeal distinction is the classic hand-rash discriminator.
• Anti-centromere → limited cutaneous disease/CREST with pulmonary arterial hypertension; anti-Scl-70 (topoisomerase I) → diffuse disease with interstitial lung disease; anti-RNA polymerase III → renal crisis. New severe hypertension plus AKI in a scleroderma patient is scleroderma renal crisis — give an ACE inhibitor; high-dose glucocorticoids are the precipitant, not the treatment.
• Adult-onset dermatomyositis mandates age-appropriate malignancy screening (anti-TIF1-γ highest risk) — the association examiners test most reliably.
• Common distractor: assuming rituximab is a last-resort agent in pemphigus. It is **FDA-approved (2018, based on the Ritux 3 trial) and endorsed by international expert consensus as first-line** alongside corticosteroids for moderate-to-severe disease.
• Vitiligo pairs with autoimmune thyroid disease — check TSH; alopecia areata shows exclamation-point hairs and responds to intralesional triamcinolone or JAK inhibitors, not antifungals.