BjALP1 plays an important role in the anti-bacterial immune response of amphioxus. (A) Survival analysis of amphioxus following feeding with S. aureus with or without the anti-bjALP1 antibody. (B) Survival analysis of amphioxus following feeding with V. anguillarum with or without the anti-bjALP1 antibody. Purified IgGs from preimmunized mouse serum were used as the control. (C–F) Section H&E staining of amphioxus after feeding with normal IgG (C), the anti-bjALP1 antibody (D), S. aureus and normal IgG (E), and S. aureus and anti-bjALP1 antibody (F). All animals were collected at 72 h after oral infection. Ten animals were used for each treatment. The representative results of three experiments are shown. g, gill; hc, hepatic cecum; m, muscle; nc, notochord; o, ovary; s, skin; sc, spinal cord; sp, spermary. *P < 0.05, **P < 0.01.