Relative Absence of Circulating Antigen-Reactive Cells during Oral Immunization

Adult rabbits subcutaneously immunized with bovine serum albumin (BSA) incorporated in incomplete Freund's adjuvant regularly had circulating antigenreactive cells (ARC) 14 to 21 days later. During a 3-month study period, animals immunized by oral administration of BSA in low concentrations rarely had detectable ARC despite the presence of comparable amounts of circulating anti-BSA. Discontinuing antigen administration led to diminished antibody concentrations but a persistent absence of detectable circulating ARC. Following a subcutaneous injection with BSA in incomplete Freund's adjuvant that resulted in detectable circulating ARC in non-immunized rabbits, orally immunized animals developed an increased rate of antibody synthesis without detectable ARC. The ability of lymphocytes from orally immunized animals to respond to BSA was demonstrated in 64% of these rabbits 1 week after 100 mg BSA was given intravenously after 3 months of antigen ingestion. These results and studies by others suggested that during oral immunization, ARC primarily are localized to gut-associated lymphoid tissue.