A type III and IV hypersensitivity reaction to microbial spores, animal proteins and chemicals.
a.      Farmer's lung is the prototypic disease caused by a reaction to Micropolyspora faeni.
b.     Fever, chills, dyspnea, leukocytosis may occur 4-6 hours after exposure and eventually resolve; symptoms and signs may recur on re-exposure.
c.      CXR: Acute - normal to reticulonodular pattern; Chronic - progressive fibrosis, honeycombing.
d.     BAL: Predominance of lymphocytes; increased IgG, IgM.
e.      Serum precipitins to offending antigen present.
f.       Pathology: - Interstitial alveolitis with lymphocytes and non-caseating granulomas (nonspecific); foam cells present (nonspecific)
Diagnosis: Compatible clinical picture, BAL with lymphocytes; serum precipitins; (inhalational challenge).

Therapy: Avoidance of continued inhalational exposure to causative antigen; corticosteroids in severe cases.

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