Systemic immunization of mature mice by the oral route.

1. Mice of several strains which are susceptible to the induction of oral tolerance by a single gavage with 20 mg of ovalbumin (Ova) when young adults (7-8 weeks old) become less susceptible or refractory to tolerance induction when mature (20-40 weeks old). The antibody-forming capacity of these mature animals remains invariant compared to young adults (8-10 weeks old). 2. Mature mice of several strains display significant serum antibody responses to 3 gavages (days 0, 7 and 28) with Ova; as assessed by ELISA titers, these responses are similar in magnitude to those elicited by standard ip immunization with small doses of Ova. 3. In mature H-III mice, gavages on days 0, 7 and 28 induced significant antibody formation. On the contrary, the ingestion of the same amounts of Ova on days 0, 7 and 28 induced oral tolerance. Concomitant gavage with saline on the days of Ova ingestion failed to inhibit tolerance induction. 4. In H-III mice displaying circulating antibodies induced by repeated gavage with Ova, additional ip injections of Ova failed to increase the antibody titers.