Levels of IgG1 anti-NP antibodies in immunized (○) IL11R−/− and (•) IL11Ra+/+ mice. Six mice of each genotype were immunized at day 0 and the response was followed by serum ELISA at weekly intervals. The mice were boosted with a soluble form of the antigen 35 days after primary immunization and the secondary response measured after 7 and 14 days. The geometric mean of each group at each time point is indicated.