**Abstract**

Repeated exposures to Saccharopolyspora rectivirgula in some individuals can lead to a hypersensitivity reaction where a pro-inflammatory feedback loop can occur in the interstitial space in the alveoli of the lungs that can ultimately lead to granuloma formation and fibrosis, referred to as Hypersensitivity pneumonitis or Farmer's Lung Disease. The pathogenesis of FLD is complex and incompletely understood. *S. rectivirgula* induces an immune response, triggering neutrophil influx into the lung followed by lymphocyte influx of CD8+ and CD4+ T cells. The cytokine IL17A has been shown to be critical for the development of *S. rectivirgula* induced Hypersensitivity pneumonitis. This chapter will review the immune response leading to the development of *S. rectivirgula* induced Hypersensitivity pneumonitis.

**Keywords:** *Saccharopolyspora rectivirgula*, hypersensitivity pneumonitis, farmer's lung disease, lung inflammation, immune response
