C3 and infection: To study the mRNA expression levels of immune-related genes and cell markers in trout skin after infection, we measured 12 immune-related genes including the cytokines (interleukin 2, 11 and 22; tumor necrosis factor α, interferon α, and interferon regulatory factor 1-2), chemokine gene (chemokine-like 19), complement 3 (C3), poly immunoglobulins receptor (pIgR), and immunoglobulin heavy chain genes (IgT, IgM, and IgD) (Figure 2A; primers used in this study are shown in Table 1) by qRT-PCR.