Immunolabeling of PrPSc in the brains and retropharyngeal lymph nodes of scrapie infected donor sheep. Widespread immunolabeling of PrPSc (dark red) deposits was observed in the brains (A, B) and retropharyngeal lymph node follicles (D, E) of preclinical and clinical scrapie infected donor animals after labeling tissues with anti-prion mAbs, respectively. No PrPSc immunolabeling was observed in the brain from a sheep without scrapie labeled with the same anti-prion mAbs (C). Immunolabeling of retropharyngeal lymph nodes section from the same scrapie donor sheep (E) block labeled with isotype-matched control mAb (F) shows no non-specific labeling. Scale bars = 50 μm.