Hypersensitivity pneumonitis (HP) is a lung disease caused by an immune overreaction to inhaled particles, most commonly mold spores, bird proteins, or certain industrial chemicals. Unlike allergies that affect your nose or sinuses, HP targets the deep tissue of the lungs, causing inflammation that can lead to permanent scarring if the exposure continues. It’s sometimes called “bird fancier’s lung” or “farmer’s lung” depending on the trigger, but the underlying process is the same regardless of the source.
How the Immune Reaction Works
When you first breathe in a triggering substance, your immune system produces antibodies against it. This initial response is an antibody-driven reaction: your body creates proteins that bind to the inhaled particles and form clusters in the lung tissue, sparking inflammation. In the early stages, doctors can often detect high levels of these specific antibodies in a blood test.
With repeated exposure, the immune response shifts. Instead of relying on antibodies, your body sends specialized immune cells directly into the lung tissue. These cells release signals that activate other immune cells and form tiny clusters of inflammation called granulomas. This cell-driven response is what causes the progressive damage seen in chronic HP. The longer the exposure continues, the more aggressive the immune attack becomes, with increasing numbers of immune cells migrating into the lungs.
Common Triggers
HP can be caused by hundreds of different inhaled substances, but most cases trace back to a few categories. Bird droppings, feathers, and the fine dust from down bedding are among the most frequent culprits. People who keep pigeons, parakeets, or chickens are at particular risk. Mold and fungal spores are the other major category, especially molds that grow in hay, grain, compost, hot tubs, and poorly ventilated humidifiers. Certain industrial chemicals and dusts can also trigger the condition in occupational settings.
The tricky part is that not everyone exposed to these substances develops HP. Only a fraction of people who work with moldy hay or keep birds will ever have an immune reaction severe enough to cause lung disease. Why some people’s immune systems overreact while others don’t remains poorly understood.
Symptoms of Acute HP
Acute HP comes on within a few hours of breathing in the trigger and typically lasts hours to days. It can feel a lot like the flu: shortness of breath, a dry cough, chest tightness, chills, fever, fatigue, and muscle aches. Because the symptoms mimic a respiratory infection, many people don’t connect them to an environmental exposure. A hallmark clue is that symptoms appear after a specific activity (cleaning a chicken coop, turning compost, visiting a home with a bird) and improve when you’re away from that environment.
Symptoms of Chronic HP
Chronic HP develops slowly, often over months or years of low-level exposure. The symptoms are subtler and more insidious: gradually worsening shortness of breath (especially during physical activity), a persistent cough, fatigue, and unexplained weight loss. Some people develop clubbing, a change in the shape of the fingertips and toenails where they become rounder and wider. Because these symptoms overlap with many other lung conditions, chronic HP is frequently misdiagnosed or diagnosed late.
Fibrotic vs. Non-Fibrotic HP
Current guidelines from the American Thoracic Society classify HP into two main categories: fibrotic and non-fibrotic. Non-fibrotic HP means the lungs are inflamed but haven’t yet developed permanent scarring. This form is generally reversible if the trigger is identified and removed. Fibrotic HP means scar tissue has already formed in the lung, which is irreversible and can progressively worsen even after exposure stops. The distinction matters enormously for prognosis. Fibrotic HP behaves more like other scarring lung diseases and carries a significantly worse outlook.
How HP Is Diagnosed
Diagnosis usually involves a combination of exposure history, imaging, and sometimes additional testing. High-resolution CT scans of the chest are the primary imaging tool. In acute and subacute HP, these scans typically show hazy patches called ground-glass opacities and tiny nodules scattered through the mid and upper lung zones. A pattern called mosaic attenuation, where patches of the lung appear different densities because some areas are trapping air, is a characteristic finding. A pattern known as the “headcheese sign,” where ground-glass opacities, air trapping, and normal lung sit side by side with sharp borders, is relatively specific to HP.
Distinguishing HP from other scarring lung diseases like idiopathic pulmonary fibrosis (IPF) can be challenging. CT scans can correctly tell them apart in most cases, but not all. IPF tends to cause honeycombing (a pattern of small cystic spaces) concentrated in the lower and outer portions of the lungs, while HP more often affects the upper and middle lung zones. Chronic HP, however, can sometimes look identical to IPF on imaging, which is why lung biopsy remains the gold standard when the diagnosis is uncertain.
A procedure called bronchoalveolar lavage, where a small amount of fluid is washed into the lungs and then collected, can also help. Finding that 30% or more of the cells in this fluid are lymphocytes (a type of immune cell) increases diagnostic confidence for HP. Blood tests for specific antibodies against suspected triggers can support the diagnosis as well, though a positive antibody test alone only shows exposure, not necessarily disease.
Treatment and Management
The single most important step in treating HP is identifying and avoiding the trigger. For many people with non-fibrotic HP, removing the source of exposure is enough for the lungs to recover on their own. This might mean rehoming a pet bird, fixing a moisture problem in the home, switching to synthetic bedding, or wearing a respiratory mask during certain work activities. Studies of farmers with HP have shown that wearing a dust mask during farming work effectively prevents recurrence.
When symptoms are significant or lung function is declining, corticosteroids are the first-line medication. These drugs suppress the immune response in the lungs and reduce inflammation. Treatment typically starts at a moderate dose, then tapers gradually over weeks to a lower maintenance level. Most people notice improvement in symptoms within days to weeks of starting treatment, though the speed of recovery depends on how much damage has already occurred.
For people with fibrotic HP who continue to worsen despite corticosteroids, or who can’t tolerate the side effects of long-term steroid use, additional immune-suppressing medications may be added. These are used as “steroid-sparing” agents to allow a lower steroid dose while still controlling the disease. Despite limited high-quality evidence for these treatments, they remain the most commonly used approach for progressive fibrotic HP.
Living With HP
For people with non-fibrotic HP who successfully avoid their trigger, the outlook is generally good. Lung function can return to normal, and symptoms resolve completely in many cases. The challenge is that identifying the trigger isn’t always straightforward, and complete avoidance isn’t always practical, especially in occupational settings.
Fibrotic HP is a different situation. The scar tissue already present won’t reverse, and in some cases the scarring continues to progress even after the trigger is removed. These patients need ongoing monitoring of lung function and may eventually require supplemental oxygen or, in severe cases, evaluation for lung transplant. Early diagnosis makes a meaningful difference, which is why connecting recurring respiratory symptoms to environmental exposures is so important. If you notice that breathing problems repeatedly improve when you leave a particular environment and return when you go back, that pattern itself is a valuable diagnostic clue worth investigating.