Furthermore, dietary 30, 60, 120 mg/kg Zn–Met supplementation noticeably increased the thymus index at 2 days post-infection (2 DPI) and 8 DPI (p < 0.05), and 120 mg/kg Zn–Met enhanced the serum IgA at 2 DPI and IgA, IgG, IgM, C3 at 8 DPI (p < 0.05). The gene discussed is C3; the disease is infection.