Antibody levels in APEC- and mock-infected birds following primary infection and after homologous re-challenge. The levels were measured by ELISA using different antigens; IgY levels against soluble χ 7122nalR antigens (A), IgY levels against O78 LPS purified from χ 7122nalR(B) and IgA levels measured against soluble χ 7122nalR antigens (C). At 0, 7, 14, 21, and 28 days post-primary infection, five birds per group were sampled and ten birds per group were sampled after re-challenge (day 42). The data in panels B and C reflect levels at post-mortem examination on day 42, one day after re-challenge. * = P ≤ 0.05; ** = P ≤ 0.01.