Immunobullous Disorders
Blistering diseases look like a list of names attached to antibodies. They become a single reasoning exercise once you ask one question.
At what level does the skin split?
The epidermis is held together by desmosomes between keratinocytes, and it is anchored to the dermis by hemidesmosomes at the basement membrane zone. Autoantibodies against the first group split the epidermis internally; autoantibodies against the second group lift the whole epidermis off the dermis.
Everything clinically observable follows from that depth.
| Feature | Intraepidermal split | Subepidermal split |
|---|---|---|
| Prototype | Pemphigus | Pemphigoid |
| Blister roof | A few cell layers, fragile | Full-thickness epidermis, tough |
| Blister appearance | Flaccid, ruptures easily, erosions | Tense, intact, may be haemorrhagic |
| Nikolsky sign | Positive | Negative |
| Mucosa | Commonly involved | Usually spared |
| Age | Middle age | Elderly |
| Prognosis untreated | Fatal | Chronic, self-limiting over years |
A second idea explains the rest: which antigen is targeted determines which body site is affected, because the anchoring proteins are not distributed evenly.
1. Desmoglein Compensation
Pemphigus antibodies target desmogleins, the transmembrane adhesion proteins of desmosomes. Two matter.
Desmoglein 3 is expressed throughout mucosal epithelium but only in the deeper epidermis of skin. Desmoglein 1 is expressed throughout the epidermis, most strongly superficially, and only weakly in mucosa.
Where both are present, one can compensate for the loss of the other. That single fact predicts the entire clinical picture.
| Antibody profile | Mucosa | Skin | Disease |
|---|---|---|---|
| Anti-Dsg3 only | Blisters, since Dsg1 is too weak to compensate | Intact, since Dsg1 compensates | Mucosal-dominant pemphigus vulgaris |
| Anti-Dsg3 and anti-Dsg1 | Blisters | Blisters | Mucocutaneous pemphigus vulgaris |
| Anti-Dsg1 only | Intact, since Dsg3 compensates | Superficial blisters | Pemphigus foliaceus |
This is why pemphigus foliaceus never involves mucosa and why pemphigus vulgaris almost always begins in the mouth, often months before any skin lesion.
It also explains the depth of the split. Pemphigus vulgaris splits suprabasally, just above the basal layer where Dsg3 lives, leaving a characteristic row of basal cells still attached to the dermis, described as a tombstone appearance. Pemphigus foliaceus splits high, in the subcorneal layer.
2. Pemphigus
Pemphigus vulgaris typically presents in middle age with painful oral erosions that will not heal, and intact blisters are rarely seen because they rupture almost immediately.
Nikolsky sign is the shearing off of apparently normal skin under firm lateral pressure, indicating acantholysis in surrounding clinically uninvolved skin. Bulla spread sign, or Asboe-Hansen sign, is the extension of an existing blister when pressure is applied to its top.
Histology shows acantholysis with a suprabasal split. Direct immunofluorescence shows intercellular IgG and C3 in a fishnet or chicken-wire pattern around keratinocytes.
Direct immunofluorescence is performed on perilesional skin, not on the blister itself, since the roof has already separated and the immunoreactants are lost.
Variants worth separating
Pemphigus foliaceus produces scaly crusted erosions on seborrhoeic areas with no mucosal disease, and its endemic form is fogo selvagem in Brazil.
Paraneoplastic pemphigus is the one that must not be missed. It presents with severe intractable stomatitis often with lichenoid skin lesions, associates with lymphoproliferative disease including chronic lymphocytic leukaemia and Castleman disease, and can involve respiratory epithelium producing bronchiolitis obliterans, which is the usual cause of death.
IgA pemphigus shows intercellular IgA rather than IgG and behaves more like a neutrophilic dermatosis.
Treatment
Rituximab has moved to first-line therapy in pemphigus vulgaris, and this is the most important change in the subject.
The Ritux 3 trial showed rituximab combined with short-term prednisolone to be superior to prednisolone alone, and long-term follow-up published subsequently reported sustained corticosteroid-free remission in a substantial proportion of first-line patients.
The mechanism fits the disease exactly: pemphigus is a B-cell-driven autoantibody disease, and depleting CD20-positive B cells removes the source of the antibody rather than merely suppressing inflammation.
The practical gain is steroid sparing. Conventional treatment required prolonged high-dose corticosteroid with its full burden of diabetes, osteoporosis, infection and myopathy, and first-line rituximab reduces both cumulative steroid dose and adverse events.
3. Bullous Pemphigoid
Bullous pemphigoid is the commonest autoimmune blistering disease, and it is a disease of the elderly.
Antibodies target BP180, also called collagen XVII, and BP230, both components of the hemidesmosome. BP180 is transmembrane and is the pathogenically important one; BP230 is intracellular.
The split is therefore below the entire epidermis, so the blister roof is full-thickness skin. Blisters are tense, often large, sometimes haemorrhagic, and they persist for days. Nikolsky sign is negative.
A prodromal phase of intense itch with urticarial or eczematous plaques, sometimes lasting weeks or months before any blister appears, is common and frequently misdiagnosed.
Direct immunofluorescence shows linear IgG and C3 along the basement membrane zone, the linear pattern contrasting with the fishnet of pemphigus.
Mucosal involvement is uncommon and mild, and untreated disease is chronic and often self-limiting over years rather than fatal, which is why treatment can be less aggressive than in pemphigus. Potent topical corticosteroid is effective in localised disease and, in trial data, compares well with systemic steroid.
The drug association that matters in India
Dipeptidyl peptidase-4 inhibitors, the gliptins, carry the highest drug-associated risk of bullous pemphigoid of any drug class, with vildagliptin most strongly implicated.
This matters disproportionately in India, where gliptins are very widely prescribed for type 2 diabetes. The drug-associated cases often show a non-inflammatory phenotype with less erythema, and they target different BP180 epitopes.
Any elderly diabetic on a gliptin who develops itch or blisters should have the drug reviewed, since withdrawal alone can lead to resolution.
4. The Other Subepidermal Diseases
| Disease | Antigen | Distinguishing feature |
|---|---|---|
| Mucous membrane pemphigoid | BP180, laminin 332 | Scarring; ocular symblepharon and blindness |
| Pemphigoid gestationis | BP180 | Third trimester, starts periumbilically |
| Dermatitis herpetiformis | Epidermal transglutaminase | Intensely itchy grouped vesicles on extensors |
| Linear IgA disease | LAD-1 | String of pearls; vancomycin-induced in adults |
| Epidermolysis bullosa acquisita | Type VII collagen | Trauma-prone sites, scarring, milia |
Mucous membrane pemphigoid scars, and that single word governs its management. Conjunctival involvement produces symblepharon, entropion, trichiasis and eventually blindness, so ophthalmological review is mandatory rather than optional.
Pemphigoid gestationis begins periumbilically in the third trimester, may flare at delivery, and can produce transient blistering in the neonate through transplacental antibody.
Dermatitis herpetiformis is the skin expression of coeliac disease. The itch is severe and the vesicles are usually excoriated before they are seen. Direct immunofluorescence shows granular IgA in the dermal papillae, distinguishing it from the linear IgA of linear IgA disease.
Dapsone produces dramatic symptomatic relief within days, but a gluten-free diet is the treatment of the underlying disease and is what controls the enteropathy and the long-term lymphoma risk.
5. Making the Diagnosis
Three investigations answer different questions, and confusing them is a common error.
Histopathology from an intact early blister shows the level of the split and whether acantholysis is present.
Direct immunofluorescence on perilesional skin shows what antibody is deposited and in what pattern: fishnet intercellular IgG in pemphigus, linear IgG and C3 at the basement membrane in pemphigoid, granular IgA in dermal papillae in dermatitis herpetiformis.
Indirect immunofluorescence and ELISA on serum detect circulating antibody and, importantly, titres correlate with disease activity in pemphigus, so they are used to monitor treatment and predict relapse.
Salt-split skin separates the subepidermal diseases from one another. Incubating skin in sodium chloride splits it through the lamina lucida, and the antibody then localises either to the roof, indicating BP180 or BP230 in bullous pemphigoid, or to the floor, indicating type VII collagen in epidermolysis bullosa acquisita.
The elegance of the salt-split technique is that it uses the disease's own logic: if the antibody target lies above the lamina lucida it travels with the roof, and if it lies below it stays with the floor. The clinical distinction matters because epidermolysis bullosa acquisita responds poorly to the treatment that controls pemphigoid.
6. Principles of Treatment
Three ideas govern treatment across the group, and they are more useful than any drug list.
Match the aggression of treatment to the natural history. Untreated pemphigus was fatal, so it justifies immunosuppression that carries real risk. Bullous pemphigoid is chronic and often remits over years, so potent topical corticosteroid alone is frequently sufficient and compares well with systemic steroid in trial data.
Remove the source rather than suppressing the effect where possible. Rituximab depletes the CD20-positive B cells producing the autoantibody, which is why it produces durable remission rather than requiring indefinite suppression. This is the reasoning behind its move to first-line use in pemphigus.
Look for a trigger before committing to long-term immunosuppression. Gliptins in bullous pemphigoid, vancomycin in linear IgA disease, and gluten in dermatitis herpetiformis are all removable causes, and removing them can make immunosuppression unnecessary.
The unglamorous parts that decide outcome
Denuded skin loses fluid, protein and heat and admits bacteria, so the mortality in severe pemphigus historically came from sepsis and metabolic disturbance rather than from the blisters themselves.
Oral erosions prevent eating, so nutritional support and analgesia are treatment rather than comfort measures. Ocular involvement in mucous membrane pemphigoid needs a specialist reviewing the eye, because scarring is silent and irreversible.
And every patient on prolonged corticosteroid needs bone protection, glycaemic monitoring and consideration of prophylaxis against opportunistic infection, which in India includes screening for latent tuberculosis and for strongyloidiasis before immunosuppression.
7. What Else Blisters
Two non-immunobullous conditions must be excluded because their management is entirely different.
Stevens-Johnson syndrome and toxic epidermal necrolysis are drug reactions with full-thickness epidermal necrosis. They have a positive Nikolsky sign, which is why they are confused with pemphigus, but the onset is acute over days, there is prominent mucosal and ocular involvement, and the histology shows necrosis rather than acantholysis.
Staphylococcal scalded skin syndrome results from exfoliative toxin cleaving desmoglein 1, which is why it produces a superficial subcorneal split identical in level to pemphigus foliaceus. It spares mucosa for exactly the same reason: desmoglein 3 compensates there.
That last point is a rare instance of a toxin and an autoantibody producing an identical clinical picture through the same molecular target.
Blistering in children
Three additional possibilities dominate paediatric practice and are separated on distribution and immunofluorescence.
Chronic bullous disease of childhood is linear IgA disease presenting before puberty. New blisters form at the edge of resolving annular lesions, producing the string of pearls or cluster of jewels appearance, typically around the perineum and lower trunk. Direct immunofluorescence shows linear IgA, and dapsone is effective.
Hereditary epidermolysis bullosa is a group of genetic defects in structural proteins, not an autoimmune disease, so immunofluorescence for autoantibodies is negative. Blistering follows minor friction from birth, and the level of the defect determines severity: simplex within the basal keratinocyte, junctional within the lamina lucida, and dystrophic below it in type VII collagen, which is the form that scars and fuses digits.
Bullous impetigo is localised staphylococcal disease producing flaccid blisters with a collarette of scale, and it shares its mechanism with staphylococcal scalded skin syndrome.
The practical rule is that blistering from birth or early infancy with a family history is genetic until proved otherwise, and blistering appearing later in an otherwise well child is more likely infective or immunobullous.
8. Worked Examples
Example 1. A 48-year-old has six weeks of painful oral erosions and now flaccid blisters on the trunk that rupture leaving raw areas. Firm lateral pressure on normal-looking skin shears the epidermis away. What is the diagnosis and which investigations confirm it?
Pemphigus vulgaris. Painful oral erosions preceding skin disease, flaccid blisters and a positive Nikolsky sign all indicate an intraepidermal split with acantholysis extending into clinically normal skin.
Confirm with three tests answering different questions. Histopathology of an intact early blister shows a suprabasal split with a tombstone row of retained basal cells. Direct immunofluorescence on perilesional skin shows intercellular IgG and C3 in a fishnet pattern. Serum ELISA detects anti-desmoglein 3 with or without anti-desmoglein 1, and the titre can then be used to monitor activity.
Example 2. A 78-year-old diabetic on vildagliptin has three months of intense itch with urticarial plaques, and now tense blisters on the limbs that remain intact for days. Nikolsky sign is negative. What is the diagnosis and what specific action is required?
Bullous pemphigoid. Tense blisters with a negative Nikolsky sign indicate a subepidermal split, and the long prodrome of itch and urticarial plaques before blistering is characteristic and often misdiagnosed as eczema.
The specific action is to review the vildagliptin. Dipeptidyl peptidase-4 inhibitors carry the highest drug-associated risk of bullous pemphigoid of any class, vildagliptin most strongly, and this is of particular importance in India where gliptins are heavily prescribed. Withdrawal of the drug, with substitution of an alternative agent, can lead to resolution and should be arranged alongside treatment with potent topical or systemic corticosteroid.
Example 3. Explain why pemphigus foliaceus never involves the mouth while pemphigus vulgaris almost always starts there.
Desmoglein compensation. Desmoglein 3 is expressed throughout mucosal epithelium but only in the deeper epidermis, while desmoglein 1 is expressed throughout the epidermis, most strongly superficially, and only weakly in mucosa.
Pemphigus foliaceus antibodies target desmoglein 1 alone. In skin, the superficial epidermis depends on desmoglein 1 with little desmoglein 3 to compensate, so it splits subcorneally. In mucosa, abundant desmoglein 3 compensates fully for the loss of the little desmoglein 1 present, so the mucosa remains intact.
Pemphigus vulgaris antibodies target desmoglein 3. In mucosa there is too little desmoglein 1 to compensate, so blistering occurs. In skin, desmoglein 1 compensates superficially, which is why mucosal-dominant disease can exist with entirely normal skin until anti-desmoglein 1 antibodies also develop.
Example 4. A 30-year-old has intensely itchy grouped vesicles on the elbows, knees and buttocks, mostly excoriated. What is the diagnosis, which immunofluorescence pattern confirms it, and what are the two components of treatment?
Dermatitis herpetiformis, the cutaneous expression of gluten sensitivity. The distribution on extensor surfaces and buttocks with severe itch is characteristic, and intact vesicles are often absent because they are scratched away.
Direct immunofluorescence on perilesional skin shows granular IgA deposits in the dermal papillae, which distinguishes it from linear IgA disease where the deposition is linear along the basement membrane.
Treatment has two separate components. Dapsone controls symptoms dramatically within days but does nothing for the enteropathy. A strict gluten-free diet treats the underlying coeliac disease, is what allows dapsone to be withdrawn eventually, and addresses the long-term risk of small bowel lymphoma.
Example 5. A patient with a blistering disease has a positive Nikolsky sign, extensive mucosal ulceration and skin detachment, and became unwell over four days after starting a new drug. Why is this probably not pemphigus?
Because the tempo and the trigger are wrong. Pemphigus develops over weeks to months, characteristically beginning with oral erosions that persist and only later producing skin blisters. Onset over four days following a new drug points strongly to toxic epidermal necrolysis.
The Nikolsky sign is positive in both, which is exactly why they are confused, but the mechanism differs. In pemphigus the epidermis splits internally through acantholysis, whereas in toxic epidermal necrolysis the full thickness of the epidermis undergoes necrosis and detaches at the dermo-epidermal junction.
Histology separates them decisively, showing acantholysis in one and full-thickness keratinocyte necrosis in the other. Management differs completely: immediate withdrawal of the culprit drug and supportive care in a burns or high-dependency setting, rather than immunosuppression.
Summary
Ask at what level the skin splits; everything else follows.
Intraepidermal split gives flaccid blisters, positive Nikolsky, mucosal disease and a worse prognosis.
Subepidermal split gives tense blisters, negative Nikolsky and usually spared mucosa.
Desmoglein 3 dominates mucosa; desmoglein 1 dominates superficial skin.
Compensation explains why foliaceus spares mucosa and vulgaris begins in the mouth.
Pemphigus vulgaris splits suprabasally, leaving a tombstone row of basal cells.
Direct immunofluorescence shows fishnet intercellular IgG in pemphigus.
Perilesional skin, not the blister, is sampled for immunofluorescence.
Paraneoplastic pemphigus associates with lymphoproliferative disease and can cause bronchiolitis obliterans.
Rituximab is now first-line in pemphigus vulgaris, on the Ritux 3 trial and its follow-up.
Bullous pemphigoid targets BP180 and BP230 and affects the elderly.
Pemphigoid shows linear IgG and C3 at the basement membrane zone.
An itchy urticarial prodrome can precede pemphigoid blisters by months.
Gliptins, especially vildagliptin, carry the highest drug-associated risk of pemphigoid.
Mucous membrane pemphigoid scars and can blind through symblepharon.
Pemphigoid gestationis starts periumbilically in the third trimester.
Dermatitis herpetiformis shows granular IgA in dermal papillae and responds to dapsone.
A gluten-free diet, not dapsone, treats the underlying coeliac disease.
Salt-split skin puts pemphigoid antibody on the roof and EBA antibody on the floor.
Toxic epidermal necrolysis also has a positive Nikolsky sign but shows necrosis, not acantholysis.
Staphylococcal scalded skin syndrome cleaves desmoglein 1, mimicking pemphigus foliaceus exactly.
