Hypersensitivity pneumonitis (HP) is a lung condition caused by an immune system reaction to inhaled substances, often environmental agents, that the body mistakenly identifies as harmful. When a person breathes in these substances repeatedly or in large amounts, their lungs can become inflamed. This inflammation primarily affects the tiny air sacs in the lungs, called alveoli, where oxygen is transferred into the blood. Over time, if exposure continues, this inflammation can lead to lung tissue damage and scarring, which can impair breathing and oxygen exchange.
The substances that trigger HP are usually organic particles, such as molds, bacteria, animal proteins, or certain chemicals. These particles are often found in specific environments or occupations. For example, people who work with birds can develop “bird fancier’s lung,” a form of HP caused by inhaling proteins from bird droppings or feather dust. The dust contains oils and proteins from the birds’ preen gland, which coat their feathers and can become airborne. Similarly, farmers exposed to moldy hay or grain dust can develop HP, sometimes called “farmer’s lung.” Other triggers include exposure to humidifiers, hot tubs, or air conditioning systems contaminated with certain bacteria or fungi.
The immune system’s reaction in HP is complex. When the lungs inhale these antigens, the immune system mounts a response involving inflammation and the activation of immune cells. This response is meant to protect the body but ends up damaging lung tissue. The inflammation can cause symptoms like breathlessness, coughing, fatigue, and sometimes weight loss. If the exposure is ongoing, the inflammation can become chronic, leading to fibrosis, which is thickening and stiffening of lung tissue that reduces lung function.
Not everyone exposed to these substances develops HP. It depends on individual susceptibility, the amount and duration of exposure, and the specific type of antigen. Some people may have a genetic predisposition that makes their immune system more likely to react strongly. The condition can develop suddenly after a high level of exposure or gradually with repeated low-level exposures.
Diagnosing HP involves identifying the exposure history, symptoms, and lung changes seen on imaging tests. Doctors may also perform lung function tests and sometimes lung biopsies to confirm inflammation and fibrosis. Avoiding the triggering antigen is the most important step in managing HP. In some cases, medications like corticosteroids are used to reduce inflammation.
In essence, hypersensitivity pneumonitis is caused by the immune system’s overreaction to inhaled environmental substances, leading to lung inflammation and potentially permanent damage if exposure continues. The causes are diverse but generally involve organic dusts, molds, bacteria, or animal proteins encountered in certain environments or jobs. Understanding and avoiding these triggers is key to preventing and managing the disease.
