The immune response of pigs after immunization with F4 fimbriae. The F4-specific IgG antibody responses (A), the F4-specific IgA antibody responses (B), number of F4-specifc total antibody secreting cells (ASCs) per 3.0 × 108 peripheral blood monomorphonuclear cells (PBMCs) (C), number of F4-specific IgA+ ASCs per 3.0 × 108 PBMCs (D). Piglets were immunized with F4 fimbriae at day 0 and at 14 days post primary immunization (dppi). Data are presented as the mean ± SEM. The asterisk indicates a statistical significant difference (p < 0.05) within a group between a time point and day 0, while letters indicate a statistical significance (p < 0.05) between groups at a designated time point; (A) in comparison to the maternal-PBS group, (b) to the seronegative-oral group. ↑, immunization time points.