Interstitial Lung Disease

Hypersensitivity Pneumonitis

Signs and Symptoms
Diagnosis
Treatment

Signs and Symptoms

Hypersensitivity pneumonitis (HP) is a group of rare inflammatory interstitial lung diseases that result from repeated inhalation of certain fungal, bacterial, animal protein, or reactive chemical particles, called antigens. The body's immune reaction to these particles causes inflammation of the very small airways in the lungs, called alveoli.  If exposure to the antigen is continuous, parts of the lungs may become scarred or fibrous.

Most people breathe in the antigens that cause HP every day. However, evidence suggests that genetics and environment may play a role in the development of HP, making some people more susceptible to the disease. Changes in certain genes may predispose a person toward developing HP. In addition, in many cases, environmental factors are the leading cause of HP. For example, some bird handlers may develop HP because of the species of bird raised and the handling practices used.

HP should not be confused with the more common types of aero-allergies, which are caused by small amounts of proteins in the environment, such as dust mites, cat dander, pollen and grass. The body's response to these proteins is very different than to the proteins that cause HP. Thus, having seasonal or environmental allergies has nothing to do with having or developing HP.

HP is a very complex condition, which has been subdivided into three forms: acute, sub-acute and chronic. Symptoms differ for each form.

• The acute form of HP occurs following intermittent, heavy exposure to the antigen. Symptoms include fever, chills, fatigue, breathlessness, chest tightness and cough. They occur rather quickly after inhalation exposure to the antigen, typically within four-eight hours. If the person is then removed from the antigen and is no longer breathing it in, the syndrome usually resolves over 24 to 48 hours.
• The sub-acute form of HP is thought to occur due to chronic, low-level exposure to a causative antigen, such as in a persons home or workplace. The symptoms are much more subtle than those of the acute form, including progressive breathlessness, cough and fatigue developing over months. Patients with sub-acute HP can have episodic attacks of symptoms that resemble acute HP, such as fevers and sudden worsening of pulmonary symptoms. These attacks may correspond to intermittent heavy antigen exposure.
• The chronic form of HP, like the sub-acute form, is also thought to occur due to chronic low-level exposure with more subtle symptoms. However, patients with chronic HP often have no history of acute episodes and in some cases may develop scarring of the lungs.

Reviewed by health care specialists at UCSF Medical Center.
Last updated May 8, 2007