A type III and IV hypersensitivity reaction to microbial spores, animal proteins and chemicals.

1. Farmer's lung is the prototypic disease caused by a reaction to Micropolyspora faeni.

2. Fever, chills, dyspnea, leukocytosis may occur 4-6 hours after exposure and eventually resolve; symptoms and signs may recur on re-exposure.

3. CXR: Acute - normal to reticulonodular pattern; Chronic - progressive fibrosis, honeycombing.

4. BAL: Predominance of lymphocytes; increased IgG, IgM.

5. Serum precipitins to offending antigen present.

6. 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.