Bullae are abnormal, fluid-filled or air-filled sacs that form either on the skin or inside the lungs, and the term covers a surprisingly wide range of conditions. On the skin, a bulla is a blister larger than about half a centimeter across, filled with clear or bloody fluid. In the lungs, a bulla is a thin-walled air pocket that develops when the tiny air sacs (alveoli) break down and merge. The causes span everything from autoimmune disease and inherited gene mutations to cigarette smoke and bacterial toxins, and the treatment depends entirely on where the bulla is and what triggered it.
Skin Bullae and Lung Bullae Are Different Problems
The word “bulla” (plural: bullae) comes from Latin for “bubble,” and that image applies in both settings, but the similarity ends there. A cutaneous (skin) bulla is a raised pocket of fluid that sits within or just beneath the outer layer of skin. The clinical threshold is size: anything under half a centimeter is called a vesicle, while anything at or above that mark is a bulla. A pulmonary bulla, on the other hand, is an air-filled space within the lung that develops when the walls between alveoli are destroyed. Pulmonary bullae have thin walls, often less than a millimeter thick, and tend to sit near the lung surface rather than deep within the tissue.1European Society of Radiology. Cystic lung lesions – Differential diagnosis made easier Because the causes, symptoms, and treatments differ so much between the two, it helps to think of skin bullae and lung bullae as essentially separate conditions that happen to share a name.
What Causes Skin Bullae
Skin bullae form whenever something disrupts the bonds that hold skin cells together or anchor the outer skin layer to the tissue beneath it. That disruption can come from the immune system, from inherited genetic defects, from infections, from drugs, or simply from sustained friction.
Autoimmune Blistering Diseases
The two most studied autoimmune causes are pemphigus and bullous pemphigoid. In pemphigus, the immune system produces antibodies that attack desmogleins, the protein “rivets” that hold neighboring skin cells together. When those connections fail, cells separate from each other, a process called acantholysis, and fluid rushes in to form blisters within the upper skin layer.2PubMed Central. Mechanisms of Disease: Pemphigus and Bullous Pemphigoid The blisters tend to be fragile and rupture easily, leaving raw, painful erosions on the skin and inside the mouth.
Bullous pemphigoid works differently. Here, antibodies target proteins in structures called hemidesmosomes, which anchor the bottom of the outer skin layer to the basement membrane below. When that anchor fails, the entire outer layer lifts off, creating tense, firm blisters that are harder to pop than those of pemphigus.3PubMed Central. The pathogenesis of bullous skin diseases Bullous pemphigoid also triggers an inflammatory cascade involving complement, which means the surrounding skin is often red and intensely itchy.2PubMed Central. Mechanisms of Disease: Pemphigus and Bullous Pemphigoid Both diseases are clinically characterized by blisters and erosions of the skin or mucous membranes.4PubMed. The role of T cells in pemphigus vulgaris and bullous pemphigoid
Genetic Causes
Epidermolysis bullosa (EB) is an inherited group of disorders in which the skin is so fragile that even mild friction causes blistering. Researchers have identified mutations in at least 16 different genes linked to EB, all of which encode proteins involved in the skin’s structural anchoring system.5PubMed Central. Molecular genetic basis of epidermolysis bullosa The severity ranges widely. When the mutated gene still produces a reduced or slightly altered protein, symptoms tend to be milder. When the mutation knocks out protein production entirely, the result is severe blistering from birth, sometimes involving the mouth, esophagus, and other internal surfaces.5PubMed Central. Molecular genetic basis of epidermolysis bullosa
Infections and Toxins
Certain strains of Staphylococcus aureus produce exfoliative toxins that directly attack the upper skin layer, causing a condition known as staphylococcal scalded skin syndrome (SSSS). The toxins act specifically at the granular layer of the epidermis, producing sheets of peeling, blistered skin that looks almost like a burn.6PubMed. Clinical, microbial, and biochemical aspects of the exfoliative toxins causing staphylococcal scalded-skin syndrome SSSS predominantly affects infants and young children, who are more vulnerable because their kidneys clear the toxin more slowly and their immune systems have not yet learned to neutralize it.7PubMed Central. Exfoliative toxins of Staphylococcus aureus The condition can also cause significant dehydration and open the door for secondary infections.
Drug Reactions
Some of the most dangerous bullae are triggered by medications. Stevens-Johnson syndrome and toxic epidermal necrolysis (SJS/TEN) sit on a spectrum of severity in which widespread skin detachment and death of skin cells occurs after exposure to an offending drug or its metabolite. The immune response is complex and aggressive, and SJS/TEN carries significant risk of death.8PubMed Central. Updates in the pathogenesis of SJS/TEN Common culprits include certain antibiotics, anti-seizure medications, and nonsteroidal anti-inflammatory drugs, though any medication can potentially trigger the reaction in a susceptible person.
Friction and Burns
Ordinary friction blisters are technically bullae when they exceed that half-centimeter size threshold. The mechanical shearing forces separate skin cells at a layer called the stratum spinosum, and the resulting pocket fills with fluid similar to blood plasma but with lower protein content.9PubMed. Friction blisters. Pathophysiology, prevention and treatment Thermal burns produce bullae through direct heat damage to the same skin layers. While these are the most familiar type of blister, they are also generally the least medically concerning because the underlying skin architecture remains intact.
What Causes Lung Bullae
Pulmonary bullae almost always develop in the context of emphysema, a form of chronic obstructive pulmonary disease in which long-term inflammation destroys the walls of the alveoli.10Radiology Case Reports. Unusual case of bullous emphysema with superimposed pneumonia As adjacent air sacs lose their walls, they merge into progressively larger spaces. The evidence suggests that bullae form after the surrounding lung tissue retracts and collapses away from a weakened region, rather than through a one-way valve mechanism trapping air, as was once assumed.11Thorax. Origin and behaviour of emphysematous bullae
Cigarette smoking is the dominant risk factor, but it is not the only one. Alpha-1 antitrypsin deficiency (AATD) is an inherited condition in which the body fails to produce enough of a protein that protects lung tissue from enzymatic damage. People with certain forms of AATD, combined with environmental exposures like smoking or dust, face an accelerated path toward emphysema and bullae formation earlier in life.12PubMed Central. Hereditary alpha-1-antitrypsin deficiency and its clinical consequences Even people who carry only one copy of the defective gene (heterozygous carriers) can develop bullous emphysema in some circumstances, though this is rare and was long thought not to occur.13PubMed Central. Heterozygous Alpha-1 Antitrypsin Deficiency Causing Pulmonary Emboli and Pulmonary Bullae
Bullae tend to form near the lung’s outer surface, particularly at the apex (the top), where the mechanical forces stretching the lung are greatest. Interestingly, factors like heart motion and diaphragm movement also contribute to local differences in pressure on the lung surface, which may help explain why some locations are more vulnerable than others.14PubMed Central. Location of Ruptured Bullae in Secondary Spontaneous Pneumothorax
Symptoms Worth Knowing
Skin bullae are hard to miss visually. The hallmarks depend on the underlying cause, but the general experience includes tense or flaccid blisters, pain or itching at the blister site, and raw open wounds where blisters have ruptured. In pemphigus vulgaris, mouth sores often appear before the skin blisters do, making eating and swallowing painful. In bullous pemphigoid, intense itching can precede visible blisters by weeks, sometimes leading to a misdiagnosis of eczema or hives. Patients with epidermolysis bullosa deal with blistering after any kind of mechanical stress, and in severe forms, the repeated cycles of blistering and healing lead to scarring, contractures, and even fusion of fingers.
Lung bullae often go unnoticed for years because the surrounding lung compensates. When they grow large enough to compress healthy tissue, though, symptoms emerge: progressive shortness of breath, reduced exercise tolerance, and a chronic feeling of air hunger. The most dramatic complication is spontaneous pneumothorax, which occurs when a bulla ruptures and air leaks into the space between the lung and the chest wall, partially or fully collapsing the lung.15PubMed Central. Etiology of primary spontaneous pneumothorax A pneumothorax typically causes sudden, sharp chest pain on one side and acute breathlessness. It is a medical emergency.
How Bullae Are Diagnosed
For skin bullae, the diagnostic workup usually starts with a physical exam and a skin biopsy. The biopsy shows where in the skin the split is occurring: within the epidermis (as in pemphigus) or below it at the basement membrane zone (as in bullous pemphigoid). But the gold standard for autoimmune blistering diseases is direct immunofluorescence (DIF), a test in which a sample of skin near a blister is stained with fluorescent dyes that light up wherever antibodies or immune proteins have been deposited. In one study of pemphigus vulgaris cases, DIF was positive in every single case, whereas standard histopathology alone was diagnostic in about 87%.16International Journal of Medical and Pharmaceutical Research. Role of Direct Immunofluorescence in the Diagnosis of Immunobullous Disorders The pattern of fluorescence matters: pemphigus shows a characteristic “fishnet” pattern of antibody deposits between cells, while bullous pemphigoid shows a linear band along the basement membrane.17PubMed Central. A Clinicopathological and Immunofluorescence Study of Intraepidermal Immunobullous Diseases
For lung bullae, high-resolution CT scanning is the primary tool. Plain chest X-rays can show large bullae, but CT provides far more detail about the size, location, and number of bullae, as well as the condition of the surrounding lung tissue. CT findings also have prognostic value: in patients who have had a spontaneous pneumothorax, the presence of blebs or bullae on CT is strongly associated with recurrence. One study found an ipsilateral recurrence rate of about 68% in patients with visible blebs or bullae, compared to roughly 6% in those without them.18PubMed. Role of blebs and bullae detected by high-resolution computed tomography and recurrent spontaneous pneumothorax That gap is significant enough to influence decisions about whether to proceed with surgery after a first episode.
Treatment for Skin Bullae
Treatment depends heavily on the cause. Friction blisters and burn blisters generally heal on their own with wound care, while SSSS requires antibiotic treatment targeting the underlying staph infection. SJS/TEN demands immediate hospitalization, drug withdrawal, and often burn-unit-level supportive care.
For autoimmune bullous diseases, the traditional approach has been systemic corticosteroids combined with steroid-sparing immunosuppressants like azathioprine or mycophenolate mofetil. This works reasonably well for many patients, but long-term steroid use carries its own serious side effects, and patients with bullous pemphigoid have expressed a strong desire to avoid corticosteroids because of those side effects.19PubMed Central. Patient Experiences of Bullous Pemphigoid: Symptoms and Health-Related Quality of Life Impacts
Targeted therapies are increasingly replacing or supplementing that approach. Rituximab, a monoclonal antibody that depletes a specific type of immune cell (B cells), has shown strong results. In a multicenter trial treating patients with refractory autoimmune bullous diseases, rituximab produced meaningful improvement in nine out of ten cases, with half achieving complete remission on minimal additional therapy.20PubMed. Rituximab therapy for refractory autoimmune bullous diseases: A multicenter, open-label, single-arm, phase 1/2 study on 10 Japanese patients Other targeted strategies include intravenous immunoglobulin infusions and immunoadsorption, which physically remove pathogenic antibodies from the blood. The appeal of these approaches is that they interfere more precisely with the disease process and carry fewer side effects than broad immunosuppression.21PubMed. Targeted Therapies for Autoimmune Bullous Diseases: Current Status
Infection is a constant concern in patients receiving immunosuppressive treatment for blistering diseases. Open skin wounds already raise the infection risk, and systemic immunosuppression compounds it. One retrospective study found that all patients taking systemic corticosteroids experienced at least one localized or systemic infection during follow-up, and roughly a third developed infections serious enough to require hospitalization or contribute to death.22Wiley Online Library (The Journal of Dermatology). Infection in autoimmune bullous diseases: a retrospective comparative study This is one of the main reasons the field has pushed toward more targeted treatments with narrower immunosuppressive profiles.
Treatment for Lung Bullae
Small, stable lung bullae that are not causing symptoms do not require treatment. When bullae are large enough to compress surrounding healthy lung tissue, cause recurrent pneumothorax, or produce significant breathlessness, intervention is warranted.
The standard surgical approach is bullectomy, in which the bullae are removed or stapled off, often paired with a procedure called pleurodesis that creates adhesion between the lung surface and the chest wall to prevent future pneumothorax. Current guidelines recommend surgery after the first recurrence of spontaneous pneumothorax.23PubMed Central. Uniportal video-assisted thoracic surgery for pneumothorax and blebs/bullae Most of these operations are now performed using video-assisted thoracoscopic surgery (VATS), a minimally invasive technique that uses small incisions and a camera. In one study of 75 patients, VATS procedures averaged about 67 minutes of operating time, roughly 49 milliliters of blood loss, and a hospital stay of under five days. Lung function improved after surgery, with forced expiratory volume rising from a mean of about 66% to about 78% of predicted values.24PubMed Central. Video-Assisted Thoracoscopic Surgery (VATS) for Spontaneous Pneumothorax and Emphysematous Bullous Lung Disease: A Study From Northern India
For patients who are too sick or too high-risk for surgery, bronchoscopic lung volume reduction using endobronchial valves has emerged as an alternative. Small one-way valves are placed into the airway feeding the bulla, blocking air from entering while allowing trapped air and secretions to drain out. In case reports, the bulla has resolved completely within weeks, and lung function has improved substantially. One patient saw a 30% gain in forced expiratory volume seven months after valve placement.25PubMed Central. Successful Treatment of Bulla with Endobronchial Valves A retrospective comparison found that endobronchial valves were associated with a lower complication rate, shorter operating time, less blood loss, and shorter hospital stays compared with traditional surgery.26PubMed. The efficacy-invasiveness trade-off: a retrospective cohort comparison of surgical operation and endobronchial valves for refractory lung bullae disease The evidence is still growing, but for appropriately selected patients, this minimally invasive option is increasingly seen as a viable alternative.27PubMed Central. Bronchoscopic lung volume reduction using an endobronchial valve to treat a huge emphysematous bullae: a case report
The Burden Beyond the Blister
Bullous diseases, particularly the chronic autoimmune and inherited forms, take a toll that goes far beyond visible skin lesions. Pain, itching, and scarring are the most direct symptoms, but the downstream effects ripple through daily life. In bullous pemphigoid, five of the six most commonly reported symptoms had average disturbance ratings above 7.5 out of 10, and patients described the disease as highly burdensome even when under treatment.19PubMed Central. Patient Experiences of Bullous Pemphigoid: Symptoms and Health-Related Quality of Life Impacts Common impacts include difficulty sleeping, restricted physical activity, social withdrawal due to the appearance of the skin, and the psychological weight of living with a chronic, visible illness.28PubMed Central. Patient Quality of Life Improvement in Bullous Disease: A Review of Primary Literature and Considerations for the Clinician
The management itself adds another layer of burden. Frequent clinic visits, lab monitoring, wound care routines, and medication side effects all demand a significant allocation of time and resources from patients and their care teams.28PubMed Central. Patient Quality of Life Improvement in Bullous Disease: A Review of Primary Literature and Considerations for the Clinician Incorporating formal quality-of-life assessment into clinical evaluations helps clinicians track how a patient is actually doing, not just how the blisters look, and adjust treatment accordingly.29PubMed Central. Quality of life in patients with bullous dermatoses
Conditions That Mimic Bullae
Not every blister is a bulla in the clinical sense, and a few conditions are commonly confused with autoimmune or emphysematous bullae. Porphyria cutanea tarda (PCT) is a metabolic disorder in which excess porphyrins accumulate in the blood and react to sunlight, producing fragile, erosive skin lesions on sun-exposed areas like the backs of the hands.30PubMed Central. Porphyria: What Is It and Who Should Be Evaluated? Under a microscope, the blistering mechanism is different: fluid-filled vacuoles form in the upper dermis rather than through the autoimmune cell-separation seen in pemphigus or the basement-membrane detachment of bullous pemphigoid.31PubMed. The morphologic events of blister formation in porphyria cutanea tarda PCT blisters tend to appear on areas that get the most sun, heal slowly, and leave scars and small white cysts called milia. The treatment is entirely different from autoimmune blistering diseases, centering on phlebotomy (regular blood removal) or antimalarial drugs to reduce porphyrin levels.
In the lung, the distinction between a bulla, a bleb, and a cyst matters for treatment decisions. Blebs are tiny air pockets less than about a centimeter across, sitting right at the lung surface or within the pleura itself. They are often attributed to higher distending pressure at the lung apex or subtle airway problems.1European Society of Radiology. Cystic lung lesions – Differential diagnosis made easier Cysts, meanwhile, can occur in conditions like lymphangioleiomyomatosis or Langerhans cell histiocytosis and have a different appearance and prognosis. Radiologists use CT to sort these out, because the wall thickness, location, and distribution pattern all provide diagnostic clues that shape what happens next.
Enzyme Damage at the Skin’s Foundation
One area of active research is the role of enzymes beyond the immune system’s antibodies in worsening blistering. Granzyme B, an enzyme released by certain immune cells, has been found at elevated levels in autoimmune blistering diseases. In laboratory experiments, granzyme B directly cleaved key anchoring proteins at the junction between the outer skin and the layer beneath it, including the alpha-6 and beta-4 integrin subunits that form part of the hemidesmosomes holding skin together.32Scientific Reports. Granzyme B is elevated in autoimmune blistering diseases and cleaves key anchoring proteins of the dermal-epidermal junction When those proteins are cut, the mechanical connection between the skin layers weakens, potentially amplifying the blistering caused by the autoantibodies alone. Importantly, specific enzyme inhibitors were able to block this cleavage in the lab. This raises the possibility that future treatments could target not just the autoantibodies themselves but also the enzymatic damage they set in motion, potentially reducing blister formation through a complementary mechanism.