Hypersensitivity pneumonitis
Clinic
- 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 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 alveoli and bronchioles within the lung.
- Hypersensitivity pneumonitis may eventually lead to interstitial lung disease.
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. They have an insidious onset of cough, progressive dyspnea, fatigue, and weight loss. This is associated with partial to complete but gradual reversibility. Avoiding any further exposure is recommended. Clubbing is observed in 50% of patients. Tachypnea, respiratory distress, and inspiratory crackles over lower lung fields often are present. In fact, hypersensitivity pneumonitis can often resemble (IPF) in terms of pulmonary fibrosis and that many patients experience hypoxemia.
Pathophysiology
- Hypersensitivity pneumonitis is caused by an exaggerated immune response (hypersensitivity).
- Type III hypersensitivity and type IV hypersensitivity can both occur depending on the cause.
- In general, acute HP is suspected to be attributed to a type III hypersensitivity while the subacute and chronic types are suspected to be caused by T cell infiltration and granuloma formation. Because different people react variably to antigen exposure, the exact mechanism is unclear but genetic and host factors are likely at play. The two hit hypothesis is often toted in the literature to explain why some people have a normal reaction to an antigenic exposure without clinical findings while others experience an exaggerated immune response. The "first hit" in the hypothesis is proposed to be genetic susceptibility and surrounding environmental factors and the "second hit" would be the introduction of the antigen into the respiratory system which causes the exaggerated immune response.