Before challenge, the serum IgA levels in the LDLP and MDLP groups significantly increased (P < 0.05) with respect to that in the CONT group. Meanwhile, the IgM concentrations in the MDLP and HDLP groups were higher (P < 0.05) than that in the CONT. However, only the IgG level in HDLP increased (P < 0.05) with respect to that in the CONT. At 24 h after ETEC challenge, the IgA and IgM contents in the LDLP, MDLP, and HDLP groups were higher (P < 0.05) than those in the CONT. At 144 h post-challenge, no significant difference in IgA, IgM, or IgG level (P > 0.05) was observed among the groups (Figure 4).