COL1A1 and hypersensitivity pneumonitis: Importantly, blocking the C5a/C5aR1 signaling either before or after the initiation of fibrosis led to significant reductions in lung tissue cell numbers, alveolitis scores, Ashcroft scores, collagen deposition, and the expression of fibrosis-related genes, including Acta2, Col1a1, Col3, and Fn1.