Impaired IgA production in Art^Δ/− mice on KLH immunization. Four series of mice were immunized with KLH orally for a total of 8 Art^Δ/− and Art^fl/− animals. (A) Percentage of Peyer patch B220⁺ cells expressing IgA, as determined by flow cytometric analysis. The decrease in IgA⁺ cells is statistically significant in series 2 and 4 (2-tailed Student t test, P = .048 and P = .024, respectively). (B) Serum levels of KLH-specific IgA and IgM were measured by ELISA (8 Art^Δ/− and 8 Art^fl). The variations in immunization efficiency were normalized by considering the ratio (anti-KLH IgA/anti-KLH IgM) for each mouse.