A bulla is a fluid-filled blister larger than 1 centimeter in diameter. It can form on the skin or inside the lungs, and the underlying cause ranges from something as simple as friction to serious autoimmune or respiratory conditions. The smaller version of the same thing, a vesicle, measures under 1 centimeter. Once a blister crosses that threshold, it’s classified as a bulla (plural: bullae).
Bullae on the Skin
Skin bullae are raised, dome-shaped pockets filled with fluid that sits between layers of skin. They can appear anywhere on the body and vary widely in size, from just over a centimeter to several centimeters across. The fluid inside is typically clear and watery (serous), but it can also be tinged with blood (hemorrhagic), which signals deeper tissue involvement. Clear bullae generally mean the separation happened in the outermost skin layer and the underlying tissue is intact. Blood-tinged bullae indicate the split occurred deeper, disrupting small blood vessels beneath the surface.
The most common everyday cause is friction or a burn. A second-degree burn, badly fitting shoes, or repeated rubbing can all produce bullae. Contact with certain chemicals does the same thing. These types usually heal on their own once the irritation stops, though larger ones may need to be drained to prevent infection.
Autoimmune Bullae
When large, tense blisters appear without an obvious injury, an autoimmune condition is often the cause. Bullous pemphigoid is the most well-known example. The immune system mistakenly attacks proteins that hold the outer and inner layers of skin together, causing them to separate and fill with fluid. The blisters tend to show up on the trunk, inner arms, groin, and lower abdomen, and they’re often intensely itchy.
Bullous pemphigoid most commonly affects people over 70. In its early stages, it may not produce visible blisters at all. Instead, it can look like hives or eczema with itching that seems out of proportion to what’s visible on the skin. This “pre-bullous” phase can make it tricky to recognize. Diagnosis requires a skin biopsy taken from the area next to a blister (not from the blister itself), along with blood tests that look for the specific antibodies driving the attack.
Infectious Causes
Certain infections produce bullae as a hallmark feature. Bullous impetigo, a skin infection caused almost exclusively by Staphylococcus aureus bacteria, is a prime example. The bacteria release a toxin that breaks apart the bonds holding skin cells together in the outermost layer, causing large, fragile blisters that rupture easily and leave behind raw, honey-crusted patches. It’s most common in children and spreads through direct contact.
Any bulla can become infected after it forms, regardless of the original cause. Signs that a bulla has become infected include increasing redness spreading outward from the blister, warmth, pus or cloudy fluid replacing the original clear contents, fever, and blisters that keep opening and leaking without healing.
Bullae in Diabetes
People with diabetes can develop a distinctive blistering condition called bullosis diabeticorum. These bullae appear spontaneously, without any injury, and typically show up on the feet, lower legs, hands, and forearms. The blisters are filled with clear fluid and the surrounding skin looks normal, with no redness or inflammation. The condition affects roughly 0.5% of people with diabetes in the United States and occurs twice as often in men as in women. The blisters tend to recur but generally heal on their own within a few weeks.
Bullae in the Lungs
Bullae aren’t limited to skin. In the lungs, a bulla is an air-filled pocket at least 1 centimeter across that forms when the tiny air sacs (alveoli) at the ends of the airways are destroyed and merge together. This is a hallmark of a specific type of COPD called bullous emphysema. Years of damage, usually from smoking, breaks down the walls between air sacs until large, useless pockets of trapped air replace functional lung tissue.
Small lung bullae may cause no symptoms at all and show up incidentally on a CT scan. Larger ones progressively crowd out healthy lung tissue, making it harder to breathe. Giant bullae, visible even on a standard chest X-ray, can sometimes be mistaken for a collapsed lung because both appear as large air-filled spaces on imaging. A CT scan is the most reliable way to tell them apart.
When a giant bulla takes up at least two-thirds of one side of the chest and breathing is significantly impaired, surgical removal (bullectomy) becomes an option. The goal is to let compressed but still-functional lung tissue re-expand. Giant bullae also carry a risk of rupture, which causes a pneumothorax, where air leaks into the space around the lung and compresses it.
How Bullae Are Evaluated
When you develop unexplained bullae on the skin, especially if they keep recurring or you’re over 70, the priority is figuring out the underlying cause rather than just treating the blister itself. A doctor will typically look at the distribution pattern, check whether the blisters are tense or fragile, and note whether the surrounding skin is inflamed. These details narrow down the possibilities quickly. Tense blisters on the trunk and limbs in an older adult point toward bullous pemphigoid. Fragile, easily ruptured blisters in a child suggest impetigo. Painless blisters on the lower extremities with no inflammation in someone with diabetes suggest bullosis diabeticorum.
For lung bullae, chest imaging tells most of the story. A standard chest X-ray can detect large bullae, but CT scanning picks up smaller ones and provides a clearer picture of how much healthy lung tissue remains. That information drives treatment decisions, particularly whether surgery would help or whether the remaining lung is too damaged to benefit.
Fracture bullae deserve a separate mention. After a bone fracture, particularly in the ankle or lower leg, bullae can form over the injured area within hours to days. These develop from swelling and tissue damage beneath the skin. Clear (serous) fracture bullae indicate the deeper skin layers are intact and carry a better prognosis. Hemorrhagic fracture bullae, filled with blood-tinged fluid, signal more extensive tissue disruption and can complicate surgical planning if an operation is needed to repair the fracture.