Hypersensitivity Pneumonitis

Overview
  • Hypersensitivity Pneumonitis is a disease of the lung characterized by inflammation of the alveoli and results from an inappropriate immune reaction to externally-inhaled organic antigens.
Etiology
  • A wide variety of organic antigens can result in hypersenitivity pneumonitis and exposure is often associated with particular occupations or recreational activities. Disease associated with each distinct etiological source of antigen has been given a colloquial name; for example, pathology associated with inhalation of "moldy" hay has been termed "Farmer's Lung". Alternatively, disease associated with inhalation of coffee bean dust is known as "Coffee Worker's Lung" while pathology caused by redwood sawdust is termed "Sequoiosis". A large variety of these subtypes of hypersensitivity pneumonitis exist and are too extensive to list.
Pathophysiology
  • Although the etiological sources of hypersensitivity pneumonitis are diverse, it is thought that a common pathogenic sequence ultimately results in alveolar inflammation. The basis of pathology appears to be an immune reaction to the inhaled organic antigen and likely involves both a Type III Hypersensitivity reaction as well as a delayed-type hypersensitivity reaction to the antigen. These notions are based on the facts that both immune complexes have been detected in inflamed lungs, indicating a Type III hypersensitivity, as well as granulomas, indicated a delayed-type hypersensitivity.
  • Inflammation of alveoli results in thickening of the alveolar membrane, reducing lung compliance, decreasig diffusing capacity, and yielding a restrictive pattern of pulmonary function. Patients often display hypoxemia, predominantly upon exertion, possibly due to diffusion defects and ventilation-perfusion defects. While inflammation may be very prominent in those who experience a symptomatic acute exposure, some patients display a slow insidious onset of symptomology. In these patients, low-grade inflammation results in long-term activation of healing processes that yield progressive interstitial fibrosis of the lung.
Morphology
  • Following an acute exposure, hypersensitivity pneumonitis is characterized by the presence of plasma cells and lymphocytes together with non-caseating granulomas in the pulmonary interstitium, particularly around alveoli. In patients who display a chronic natural history, lungs may appear diffusely fibrotic and may be difficult to distinguish from other causes of pulmonary fibrosis.
Clinical Consequences
  • Hypersensitivity pneumonitis may present in a variety of ways depending on the frequency and dose of exposure along with host immune factors. Some patients display an acute reaction 6 - 8 hours after a high dose exposure characterized by dyspnea, cough, fever, and other constitutional symptoms that remit after several days. In the acute syndrome, hypoxemia may be observed with exertion and more rarely at rest. In other patients that undergo years of low-dose chronic exposure, disease may manifest insidiously with dyspnea, cough, weight loss, and hypoxemia.