Hypersensitivity pneumonitis (HP) or extrinsic allergic alveolitis (EAA) is a syndrome caused by the repetitive inhalation of antigens from the environment in susceptible or sensitized people. Common antigens include molds, bacteria, bird droppings, bird feathers, agricultural dusts, bioaerosols and chemicals from paints or plastics. People affected by this type of lung inflammation (pneumonitis) are commonly exposed to the antigens by their occupations, hobbies, the environment and animals. The inhaled antigens produce a hypersensitivity immune reaction causing inflammation of the airspaces (alveoli) and small airways (bronchioles) within the lung. Hypersensitivity pneumonitis may eventually lead to interstitial lung disease. Signs and symptoms
Hypersensitivity pneumonitis (HP) can be categorized as acute, subacute, and chronic based on the duration of the illness. Acute
In the acute form of HP dose of antigen exposure tends to be very high but only for a short duration. Symptoms may develop 4–6 hours following heavy exposure to the provoking antigen. Symptoms include fever, chills, malaise, cough, chest tightness, dyspnea, rash, swelling and headache. Symptoms resolve within 12 hours to several days upon cessation of exposure. Subacute
Patients with subacute HP gradually develop a productive cough, dyspnea, fatigue, anorexia, weight loss, and pleurisy. Symptoms are similar to the acute form of the disease, but are less severe and last longer. Findings may be present in patients who have experienced repeated acute attacks. Chronic
In chronic HP, dose of the antigen tends to be low volume but for a longer duration. Patients often lack a history of acute episodes.
This is a good way to help exclude other similar lung diseases like sarcoidosis, infection and Idiopathic pulmonary fibrosis. Lung biopsy
Lung biopsies can be diagnostic in cases of chronic hypersensitivity pneumonitis, or may help to suggest the diagnosis and trigger or intensify the search for an allergen. The main feature of chronic hypersensitivity pneumonitis on lung biopsies is expansion of the interstitium by lymphocytes accompanied by an occasional multinucleated giant cell or loose granuloma.When fibrosis develops in chronic hypersensitivity pneumonitis, the differential diagnosis in lung biopsies includes the idiopathic interstitial pneumonias. This group of diseases includes usual interstitial pneumonia, non-specific interstitial pneumonia and cryptogenic organizing pneumonia, among others.The prognosis of some idiopathic interstitial pneumonias, e.g. idiopathic usual interstitial pneumonia (i.e. idiopathic pulmonary fibrosis), are very poor and the treatments of little help. This contrasts the prognosis (and treatment) for hypersensitivity pneumonitis, which is generally fairly good if the allergen is identified and exposures to it significantly reduced or eliminated. Thus, a lung biopsy, in some cases, may make a decisive difference. Serum Precipitins
Assays for serum IgGs can aid in identifying possible antigenic exposures and are used as markers of exposure  However, there use is limited in making a definitive diagnosis because serum antibody levels are often elevated in those people who are exposed to an antigen but do not have the disease. Up to 90% of people exposed to the antigen have precipitins but only 50% of similarly exposed people who are asymptomatic have the same precipitins.