C3 and scrapie: (1) Total complement activity (CH50) was higher in the brain of scrapie-infected rodents. (2) C1q was upregulated in the brain of scrapie-infected rodents. (3) Stronger C3 signals in scrapie-infected brain. C3 colocalized with astrocytes, microglia, and neurons. (4) MAC was deposited in the infected brain and colocalized with neurons.