SummaryImmunohistological study of lung biopsies from four palients with farmer's lung revealed histological evidence of vasculilis along with deposits of immunoglobulins and C3 in and around the affected vessels in the biopsies from two palients. Both these patients were exposed to mouldy hay antigens shortly before biopsy. Both these patients also had considerable numbers of eosinophils in their lung lesions. Immune‐complex mediated vasculitis is probably an early event in the pathogenesis of farmer's lung and might not be detectable in the absence of recent exposure to relevant antigens. The presence of large numbers of eosinophits in these lesions suggests a type I reaction acting as the trigger for a type 3 reactio
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