FcRn expression was required for offspring to obtain the protective factor(s) from breast milk. Pups are those identified in Table 1. Following weaning, allergic airway disease was elicited in 8 week old offspring by two immunizations with OVA-Al(OH)3 followed by challenge for 4 days with 1% aerosolized OVA (daily exposure time 60 min). Parameters of disease severity measured were (A) OVA-specific IgE titers determined by ELISA; and (B and C) distribution of leukocyte populations in the airways determined by fluorescence flow cytometry. (D) Representative lung sections after OVA-aerosol challenge. Results expressed as means ± SEM and represent 12-19 mice per group. * p ≤ 0.05, ** p ≤ 0.01, or *** p ≤ 0.001 when compared to FcRn+/-/B6naive offspring (positive controls for disease) similarly subjected to AAD. Similar differences in OVA-specific IgE titers and histopathology between groups were obtained in an independent experiment.