Introduction
Bullous pemphigoid is caused by an autoimmune reaction in which the immune system mistakenly attacks proteins that anchor the outer layer of the skin to the deeper layers beneath it. This attack weakens the connection between skin layers, allowing fluid to collect and form large, tense blisters. The condition develops through a combination of biological misdirection, age-related immune changes, and, in some people, triggering factors such as certain medications or other medical disorders. Understanding bullous pemphigoid means understanding how the immune system loses tolerance to specific skin structures and why that loss occurs in some individuals but not others.
Biological Mechanisms Behind the Condition
To understand the causes of bullous pemphigoid, it helps to know how the skin is normally held together. The top layer of the skin, the epidermis, is attached to the underlying dermis by a specialized structure called the basement membrane zone. This zone contains proteins that act like molecular fasteners. Two of the most important are BP180, also called type XVII collagen, and BP230. These proteins help keep the epidermis firmly anchored and mechanically stable.
In bullous pemphigoid, the immune system produces antibodies, usually of the IgG class, against BP180 and sometimes BP230. These antibodies bind to the basement membrane zone and trigger inflammation. Immune cells such as eosinophils, neutrophils, mast cells, and complement proteins become activated. Their inflammatory chemicals and enzymes damage the junction between the epidermis and dermis. As a result, the two layers separate, and a blister forms in the space created by that separation.
This process is not simply a surface skin problem. It is driven by a breakdown in immune tolerance, meaning the immune system no longer recognizes certain skin proteins as “self.” Why this happens is influenced by several factors, including aging, genetic susceptibility, immune system dysregulation, and environmental or medical triggers. In many cases, the disease appears after a long silent period in which autoantibodies may already be present at low levels before obvious blistering begins.
Primary Causes of Bullous pemphigoid
The most direct cause of bullous pemphigoid is autoimmunity against hemidesmosomal proteins in the skin. However, several factors are strongly associated with initiating or amplifying that autoimmune response.
1. Age-related immune change
Most cases occur in older adults, especially people over 70. Aging affects the immune system in several ways. The body becomes less efficient at preserving immune tolerance, and immune regulation becomes less precise. At the same time, older skin may undergo structural changes and accumulate damage over time, which may expose or alter target proteins such as BP180. These changes can make the immune system more likely to recognize skin components as foreign. Aging also increases the likelihood of chronic inflammation, sometimes called inflammaging, which can lower the threshold for autoimmune activity.
2. Autoantibody production against BP180 and BP230
The central biological event in bullous pemphigoid is the production of autoantibodies. BP180 is the most important target in many patients. When antibodies bind to this protein, they activate complement pathways and recruit inflammatory cells. BP230, another anchoring protein, is also targeted in some cases. The immune response may begin with BP180 and later spread to related proteins, a process called epitope spreading. This broadens the autoimmune attack and can increase disease activity. The body essentially mistakes a normal adhesion protein for an invading antigen.
3. Medication triggers
Certain drugs have been linked to bullous pemphigoid or bullous pemphigoid-like disease. Dipeptidyl peptidase-4 inhibitors, used for diabetes, are a well-known example. Some diuretics, antibiotics, immune-modifying drugs, and checkpoint inhibitor cancer therapies have also been associated with cases. These medications may alter immune signaling, modify antigen processing, or increase inflammatory activity in the skin. In some individuals, this appears to unmask a latent autoimmune tendency. The result is not a simple allergic reaction but a drug-associated shift in immune behavior that promotes autoantibody formation or tissue injury.
4. Neurologic disease association
Bullous pemphigoid is more common in people with neurologic disorders such as Parkinson disease, dementia, stroke, and multiple sclerosis. The connection is not fully understood, but one explanation is that BP180 and related proteins may be expressed not only in skin but also in nervous tissue. Damage to the nervous system may alter immune surveillance or expose shared antigens, making cross-reactive autoimmunity more likely. Another possibility is that the overall inflammatory and immune changes associated with neurologic illness contribute indirectly to loss of tolerance.
Contributing Risk Factors
Several additional factors can raise the likelihood of developing bullous pemphigoid, even if they are not direct causes on their own.
Genetic influences
Genetics appears to affect susceptibility rather than acting as a single deterministic cause. Certain HLA class II gene variants are associated with increased risk. HLA molecules help present antigens to T cells, so inherited differences in these genes can shape how the immune system recognizes BP180 or BP230. If a person’s antigen-presenting system is more likely to display these proteins in an immunogenic way, the chance of autoantibody development rises. Genetic background also influences how strongly the immune system regulates inflammation, which may help explain why only some older adults develop the disease.
Environmental exposures
Skin injury, chronic irritation, ultraviolet exposure, and physical stress to the skin may contribute to disease onset in susceptible people. These exposures can damage the basement membrane zone, potentially exposing hidden parts of BP180 or BP230 to the immune system. Repeated tissue injury can also create inflammatory signals that increase antigen presentation and activate immune cells. In this setting, normal repair processes may inadvertently promote autoimmunity.
Infections
Infections are not established as a primary cause, but they may act as immune triggers. Infection can stimulate broad immune activation and increase cytokine production. In some cases, the inflammatory environment may break tolerance to self-proteins in the skin. Molecular mimicry has also been proposed, meaning that microbial antigens may resemble human proteins closely enough to provoke cross-reactive antibodies. The evidence is not definitive, but infections may contribute to the immune disturbance that allows the disease to begin.
Hormonal and immune-regulatory changes
Hormonal influences are less clearly defined than in some other autoimmune diseases, but systemic changes related to aging, stress physiology, and sex-related immune differences may affect risk. Hormones influence immune cell activity and inflammatory balance. When those regulatory signals change, the immune system may become more prone to overreact to self-antigens. This is probably a supporting factor rather than a primary one.
Lifestyle and general health status
Indirectly, general health can influence risk by affecting immune stability and skin integrity. Frailty, malnutrition, chronic illness, and polypharmacy may all increase vulnerability. These conditions can impair tissue repair, promote inflammation, and increase exposure to medications that may trigger disease. Lifestyle factors do not usually cause bullous pemphigoid alone, but they can shape the biological environment in which it develops.
How Multiple Factors May Interact
Bullous pemphigoid usually arises from a convergence of factors rather than a single event. An older person may already have age-related weakening of immune tolerance. If that person also has genetic susceptibility, the immune system may be more easily directed toward skin adhesion proteins. A medication, infection, or skin injury may then provide the trigger that tips the balance from silent autoimmunity to active blistering disease.
These interactions matter because the immune system responds to patterns, not isolated signals. Tissue injury releases inflammatory mediators; inflammatory mediators improve antigen presentation; antigen presentation encourages T-cell activation; T cells support B cells; and B cells produce autoantibodies. Once this loop begins, complement activation and eosinophil recruitment intensify tissue damage, exposing even more antigen. That creates a self-amplifying cycle in which inflammation and autoimmunity reinforce each other.
Variations in Causes Between Individuals
The causes of bullous pemphigoid differ between individuals because the disease is shaped by each person’s unique biology and exposure history. One person may develop the condition primarily because of an autoimmune predisposition that appears with aging, while another may develop it after starting a specific medication. A third may have a neurologic disorder that seems to increase susceptibility through shared antigens or immune dysregulation.
Age is especially important because the disease is uncommon in younger adults. In older people, immune surveillance changes, the skin barrier becomes more vulnerable, and cumulative exposures are greater. Health status also matters: chronic disease, reduced mobility, and immune-altering therapies can all change the likelihood of antibody formation or inflammatory amplification. Environmental exposure varies as well. Some people experience repeated skin trauma, while others have little such exposure. The disease is therefore best understood as the result of overlapping influences rather than one universal cause.
Conditions or Disorders That Can Lead to Bullous pemphigoid
Several medical conditions are linked to bullous pemphigoid or may help trigger it. Neurologic disorders are among the most consistently associated. Parkinson disease, dementia, stroke, epilepsy, and multiple sclerosis have all been reported more frequently in people with bullous pemphigoid than in the general population. The mechanism may involve shared antigens between skin and nervous tissue, altered immune regulation after nervous system injury, or both.
Autoimmune and inflammatory disorders may also play a role by indicating a background tendency toward immune misrecognition. Patients with other autoimmune diseases are not guaranteed to develop bullous pemphigoid, but a preexisting autoimmune environment suggests that tolerance mechanisms are already altered. In such individuals, a new autoimmune process may emerge more easily.
Endocrine disorders, particularly diabetes, are also relevant because some diabetes medications have been associated with bullous pemphigoid. In these cases, the underlying diabetes itself may not be the direct cause, but the treatment can contribute to immune activation in a genetically or immunologically susceptible person. Similarly, chronic kidney disease, cancer, and other serious illnesses may increase risk by altering immune regulation, increasing medication exposure, or generating ongoing inflammatory stress.
Conclusion
Bullous pemphigoid develops when the immune system attacks proteins that normally anchor the skin’s outer layer to deeper tissue. The central biological cause is autoantibody-mediated damage, especially against BP180 and sometimes BP230, leading to separation of skin layers and blister formation. Age-related immune decline, genetic susceptibility, medication exposure, neurologic disease, and inflammatory or environmental triggers can all contribute to this process.
The condition does not usually arise from one isolated cause. Instead, it reflects an interaction between immune dysregulation, skin structure, and external or medical triggers. Understanding these mechanisms explains why bullous pemphigoid appears most often in older adults, why it is sometimes linked to specific drugs or disorders, and why its onset can vary so much from one person to another.